From 9aa6134366c802a50646d66a8f03f9e84c09679d Mon Sep 17 00:00:00 2001
From: Patrick Shriwise <pshriwise@gmail.com>
Date: Fri, 21 Jan 2022 08:47:10 -0600
Subject: [PATCH 01/11] Adding otf ww adjustment using lower bound ratio.

---
 include/openmc/particle_data.h               |  7 ++++-
 include/openmc/weight_windows.h              |  9 +++++++
 openmc/weight_windows.py                     | 27 +++++++++++++++++++-
 src/weight_windows.cpp                       | 22 ++++++++++++++++
 tests/regression_tests/weightwindows/test.py |  6 +++--
 5 files changed, 67 insertions(+), 4 deletions(-)

diff --git a/include/openmc/particle_data.h b/include/openmc/particle_data.h
index d31c20022e..128685fef2 100644
--- a/include/openmc/particle_data.h
+++ b/include/openmc/particle_data.h
@@ -309,7 +309,9 @@ private:
   int n_event_ {0}; // number of events executed in this particle's history
 
   // Weight window information
-  int n_split_ {0}; // Number of splits this particle has undergone
+  int n_split_ {0}; // Number of times this particle has been split
+  double ww_factor_ {
+    0.0}; // Particle-specific factor for on-the-fly weight window adjustment
 
 // DagMC state variables
 #ifdef DAGMC
@@ -435,6 +437,9 @@ public:
   int n_split() const { return n_split_; }
   int& n_split() { return n_split_; }
 
+  double ww_factor() const { return ww_factor_; }
+  double& ww_factor() { return ww_factor_; }
+
 #ifdef DAGMC
   moab::DagMC::RayHistory& history() { return history_; }
   Direction& last_dir() { return last_dir_; }
diff --git a/include/openmc/weight_windows.h b/include/openmc/weight_windows.h
index 9c42138901..c2ef29ae8f 100644
--- a/include/openmc/weight_windows.h
+++ b/include/openmc/weight_windows.h
@@ -52,12 +52,20 @@ extern vector<unique_ptr<WeightWindows>> weight_windows;
 struct WeightWindow {
   double lower_weight {-1}; // -1 indicates invalid state
   double upper_weight {1};
+  double max_lb_ratio {1};
   double survival_weight {0.5};
   double weight_cutoff {DEFAULT_WEIGHT_CUTOFF};
   int max_split {1};
 
   //! Whether the weight window is in a valid state
   bool is_valid() const { return lower_weight >= 0.0; }
+
+  //! Adjust the weight window by a constant factor
+  void scale(double factor)
+  {
+    lower_weight *= factor;
+    upper_weight *= factor;
+  }
 };
 
 //==============================================================================
@@ -96,6 +104,7 @@ private:
   vector<double> lower_ww_;        //!< Lower weight window bounds
   vector<double> upper_ww_;        //!< Upper weight window bounds
   double survival_ratio_ {3.0};    //!< Survival weight ratio
+  double max_lb_ratio_ {1.0}; //!< Maximum lower bound to particle weight ratio
   double weight_cutoff_ {DEFAULT_WEIGHT_CUTOFF}; //!< Weight cutoff
   int max_split_ {10};             //!< Maximum value for particle splitting
   int32_t mesh_idx_;               //!< index in meshes vector
diff --git a/openmc/weight_windows.py b/openmc/weight_windows.py
index cb9fc51b81..df46e318c0 100644
--- a/openmc/weight_windows.py
+++ b/openmc/weight_windows.py
@@ -48,6 +48,10 @@ class WeightWindows(IDManagerMixin):
     survival_ratio : float
         Ratio of the survival weight to the lower weight window bound for
         rouletting
+    max_lower_bound_ratio: float
+        Maximum allowed ratio of a weight window's lower bound to particle weight.
+        A factor will be applied to raise the weight window to the particle's weight
+        during transport if exceeded. Only applied if > 1.0. (Default: 1.0).
     max_split : int
         Maximum allowable number of particles when splitting
     weight_cutoff : float
@@ -76,6 +80,9 @@ class WeightWindows(IDManagerMixin):
     survival_ratio : float
         Ratio of the survival weight to the lower weight window bound for
         rouletting
+    max_lower_bound_ratio: float
+        Maximum allowed ratio of a weight window's lower bound to particle
+        weight. (Default: 1.0)
     max_split : int
         Maximum allowable number of particles when splitting
     weight_cutoff : float
@@ -91,7 +98,7 @@ class WeightWindows(IDManagerMixin):
 
     def __init__(self, mesh, lower_ww_bounds, upper_ww_bounds=None,
                  upper_bound_ratio=None, energy_bins=None, particle_type='neutron',
-                 survival_ratio=3, max_split=10, weight_cutoff=1.e-38, id=None):
+                 survival_ratio=3, max_lb_ratio=1.0, max_split=10, weight_cutoff=1.e-38, id=None):
         self.mesh = mesh
         self.id = id
         self.particle_type = particle_type
@@ -121,6 +128,7 @@ class WeightWindows(IDManagerMixin):
                              'do not match')
 
         self.survival_ratio = survival_ratio
+        self.max_lower_bound_ratio = max_lb_ratio
         self.max_split = max_split
         self.weight_cutoff = weight_cutoff
 
@@ -192,6 +200,16 @@ class WeightWindows(IDManagerMixin):
         cv.check_greater_than('Survival ratio', val, 1.0, True)
         self._survival_ratio = val
 
+    @property
+    def max_lower_bound_ratio(self):
+        return self._max_lower_bound_ratio
+
+    @max_lower_bound_ratio.setter
+    def max_lower_bound_ratio(self, val):
+        cv.check_type('Maximum lower bound ratio', val, Real)
+        cv.check_greater_than('Maximum lower bound ratio', val, 1.0, True)
+        self._max_lower_bound_ratio = val
+
     @property
     def max_split(self):
         return self._max_split
@@ -241,6 +259,9 @@ class WeightWindows(IDManagerMixin):
         subelement = ET.SubElement(element, 'survival_ratio')
         subelement.text = str(self.survival_ratio)
 
+        subelement = ET.SubElement(element, 'max_lower_bound_ratio')
+        subelement.text = str(self.max_lower_bound_ratio)
+
         subelement = ET.SubElement(element, 'max_split')
         subelement.text = str(self.max_split)
 
@@ -278,6 +299,7 @@ class WeightWindows(IDManagerMixin):
         ebins = [float(b) for b in get_text(elem, 'energy_bins').split()]
         particle_type = get_text(elem, 'particle_type')
         survival_ratio = float(get_text(elem, 'survival_ratio'))
+        max_lb_ratio = float(get_text(elem, 'max_lower_bound_ratio'))
         max_split = int(get_text(elem, 'max_split'))
         weight_cutoff = float(get_text(elem, 'weight_cutoff'))
         id = int(get_text(elem, 'id'))
@@ -289,6 +311,7 @@ class WeightWindows(IDManagerMixin):
             energy_bins=ebins,
             particle_type=particle_type,
             survival_ratio=survival_ratio,
+            max_lb_ratio=max_lb_ratio,
             max_split=max_split,
             weight_cutoff=weight_cutoff,
             id=id
@@ -318,6 +341,7 @@ class WeightWindows(IDManagerMixin):
         lower_ww_bounds = group['lower_ww_bounds'][()]
         upper_ww_bounds = group['upper_ww_bounds'][()]
         survival_ratio = group['survival_ratio'][()]
+        max_lb_ratio = group['max_lower_bound_ratio'][()]
         max_split = group['max_split'][()]
         weight_cutoff = group['weight_cutoff'][()]
 
@@ -328,6 +352,7 @@ class WeightWindows(IDManagerMixin):
             energy_bins=ebins,
             particle_type=ptype,
             survival_ratio=survival_ratio,
+            max_lb_ratio=max_lb_ratio,
             max_split=max_split,
             weight_cutoff=weight_cutoff,
             id=id
diff --git a/src/weight_windows.cpp b/src/weight_windows.cpp
index c3859d7d15..32886436a8 100644
--- a/src/weight_windows.cpp
+++ b/src/weight_windows.cpp
@@ -56,6 +56,19 @@ void apply_weight_windows(Particle& p)
     return;
   }
 
+  // check if particle is far above current weight window
+  // only do this if the factor is not already set on the particle and a
+  // maximum lower bound ratio is specified
+  if (p.ww_factor() == 0.0 && weight_window.max_lb_ratio > 1.0 &&
+      p.wgt() > weight_window.lower_weight * weight_window.max_lb_ratio) {
+    p.ww_factor() =
+      p.wgt() / (weight_window.lower_weight * weight_window.max_lb_ratio);
+  }
+
+  // move weight window closer to the particle weight if needed
+  if (p.ww_factor() > 1.0)
+    weight_window.scale(p.ww_factor());
+
   // if particle's weight is above the weight window split until they are within
   // the window
   if (weight > weight_window.upper_weight) {
@@ -139,6 +152,14 @@ WeightWindows::WeightWindows(pugi::xml_node node)
                   "and less than the upper to lower weight window ratio.");
   }
 
+  // get the max lower bound ratio - optional
+  if (check_for_node(node, "max_lower_bound_ratio")) {
+    max_lb_ratio_ = std::stod(get_node_value(node, "max_lower_bound_ratio"));
+    if (max_lb_ratio_ < 1.0) {
+      fatal_error("Maximum lower bound ratio must be larger than 1");
+    }
+  }
+
   // get the max split - optional
   if (check_for_node(node, "max_split")) {
     max_split_ = std::stod(get_node_value(node, "max_split"));
@@ -254,6 +275,7 @@ void WeightWindows::to_hdf5(hid_t group) const
   write_dataset(ww_group, "lower_ww_bounds", lower_ww_);
   write_dataset(ww_group, "upper_ww_bounds", upper_ww_);
   write_dataset(ww_group, "survival_ratio", survival_ratio_);
+  write_dataset(ww_group, "max_lower_bound_ratio", max_lb_ratio_);
   write_dataset(ww_group, "max_split", max_split_);
   write_dataset(ww_group, "weight_cutoff", weight_cutoff_);
   write_dataset(ww_group, "mesh", this->mesh().id_);
diff --git a/tests/regression_tests/weightwindows/test.py b/tests/regression_tests/weightwindows/test.py
index 63d773cbcd..e9e64b0466 100644
--- a/tests/regression_tests/weightwindows/test.py
+++ b/tests/regression_tests/weightwindows/test.py
@@ -92,13 +92,15 @@ def model():
                                 ww_n_lower_bnds,
                                 None,
                                 10.0,
-                                e_bnds)
+                                e_bnds,
+                                max_lb_ratio=5.0)
 
     ww_p = openmc.WeightWindows(ww_mesh,
                                 ww_p_lower_bnds,
                                 None,
                                 10.0,
-                                e_bnds)
+                                e_bnds,
+                                max_lb_ratio=5.0)
 
     model.settings.weight_windows = [ww_n, ww_p]
 

From b9bbfa5d3343b6bdbfef3d2de7394ba9f9fdcf4d Mon Sep 17 00:00:00 2001
From: Patrick Shriwise <pshriwise@gmail.com>
Date: Tue, 25 Jan 2022 17:02:07 -0600
Subject: [PATCH 02/11] Setting the max lower bound ratio when retrieving a
 weight window.

---
 src/weight_windows.cpp | 1 +
 1 file changed, 1 insertion(+)

diff --git a/src/weight_windows.cpp b/src/weight_windows.cpp
index 32886436a8..e742d48c3f 100644
--- a/src/weight_windows.cpp
+++ b/src/weight_windows.cpp
@@ -260,6 +260,7 @@ WeightWindow WeightWindows::get_weight_window(const Particle& p) const
   ww.lower_weight = lower_ww_[ww_index];
   ww.upper_weight = upper_ww_[ww_index];
   ww.survival_weight = ww.lower_weight * survival_ratio_;
+  ww.max_lb_ratio = max_lb_ratio_;
   ww.max_split = max_split_;
   ww.weight_cutoff = weight_cutoff_;
   return ww;

From 6fc769a36dd7eda1ae9e93b23d5fbf6d719ec2b6 Mon Sep 17 00:00:00 2001
From: Patrick Shriwise <pshriwise@gmail.com>
Date: Wed, 26 Jan 2022 13:41:14 -0600
Subject: [PATCH 03/11] Updating test input.

---
 tests/regression_tests/weightwindows/inputs_true.dat | 2 ++
 tests/regression_tests/weightwindows/test.py         | 4 ++--
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/tests/regression_tests/weightwindows/inputs_true.dat b/tests/regression_tests/weightwindows/inputs_true.dat
index 5459a09b92..c7720d1dfe 100644
--- a/tests/regression_tests/weightwindows/inputs_true.dat
+++ b/tests/regression_tests/weightwindows/inputs_true.dat
@@ -51,6 +51,7 @@
     <lower_ww_bounds>-1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 7.695031155172816e-14 -1.0 7.476545089278482e-06 -1.0 2.1612150425221703e-06 -1.0 -1.0 -1.0 2.1360401784344975e-18 -1.0 1.3455341306162382e-05 -1.0 3.353295403842037e-05 -1.0 3.0160758454323657e-07 -1.0 1.0262431550731535e-13 -1.0 1.6859219614058024e-19 -1.0 3.853097455417877e-06 -1.0 4.354946981329799e-05 -1.0 7.002861620919232e-08 -1.0 -1.0 -1.0 -1.0 -1.0 1.6205064883026195e-11 -1.0 1.7041669224797384e-09 -1.0 1.8747824667478637e-14 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 5.619835883512353e-14 -1.0 1.951356547084204e-15 -1.0 8.48176145671274e-10 -1.0 -1.0 -1.0 1.4508262073256659e-13 -1.0 4.109066205029878e-05 -1.0 0.00020573181450240786 -1.0 2.124999182004655e-05 -1.0 5.5379154134462336e-08 -1.0 1.5550455959178486e-06 -1.0 0.0007213722022489493 -1.0 0.007673145137236567 -1.0 0.0008175033526488423 -1.0 1.008394964750947e-06 -1.0 6.848642863465585e-07 -1.0 0.0010737081832502356 -1.0 0.007197662162700777 -1.0 0.0007151459162818186 -1.0 2.253382683081525e-06 -1.0 1.3181562352445092e-10 -1.0 1.93847081892941e-05 -1.0 0.00046029617115127556 -1.0 2.5629068330446215e-05 -1.0 5.504813693036933e-12 -1.0 -1.0 -1.0 3.3164825349503764e-16 -1.0 3.937160538176268e-07 -1.0 -1.0 -1.0 -1.0 -1.0 1.5172912057312398e-14 -1.0 6.315154082213375e-07 -1.0 3.855884234344845e-06 -1.0 1.7770136411255912e-05 -1.0 1.6421491993751715e-11 -1.0 5.97653814110781e-06 -1.0 0.001959411999677113 -1.0 0.013330619853379314 -1.0 0.0017950613169359904 -1.0 2.4306375693722205e-06 -1.0 6.922231352729155e-05 -1.0 0.08657573566388353 -1.0 -1.0 -1.0 0.0835648710074336 -1.0 0.00016419031150495913 -1.0 9.03108551826469e-05 -1.0 0.0848872967381878 -1.0 -1.0 -1.0 0.08288806458368345 -1.0 5.6949597066784976e-05 -1.0 8.037131813528501e-07 -1.0 0.0016637126543321873 -1.0 0.01664109232689093 -1.0 0.0018327593437608 -1.0 6.542700644645627e-07 -1.0 -1.0 -1.0 6.655926357459275e-08 -1.0 5.783974359937857e-06 -1.0 2.679340942769232e-06 -1.0 -1.0 -1.0 -1.0 -1.0 2.1040609012865134e-05 -1.0 2.9282097947816224e-05 -1.0 1.284759166582202e-05 -1.0 1.0092905083158804e-11 -1.0 1.3228013765525015e-05 -1.0 0.007136750029575816 -1.0 0.06539005235506369 -1.0 0.0064803314120165335 -1.0 2.6011787393916806e-06 -1.0 8.887234042637684e-05 -1.0 0.5 -1.0 -1.0 -1.0 0.4877856021170283 -1.0 0.00017699858357744463 -1.0 0.0001670700632870335 -1.0 0.4899999304038166 -1.0 -1.0 -1.0 0.48573842213878143 -1.0 0.00021534568699157805 -1.0 7.84289689494925e-06 -1.0 0.0063790654507599135 -1.0 0.06550426960512012 -1.0 0.006639379668946672 -1.0 7.754700849337902e-06 -1.0 2.7207433165796702e-11 -1.0 5.985108617124989e-06 -1.0 3.634644995026692e-05 -1.0 3.932389299316285e-06 -1.0 -1.0 -1.0 -1.0 -1.0 1.271128669411295e-07 -1.0 8.846493399850663e-06 -1.0 2.499931357628383e-07 -1.0 8.533030345699353e-17 -1.0 1.6699254277734109e-06 -1.0 0.0017541380096893788 -1.0 0.015148978428271613 -1.0 0.0016279794552427574 -1.0 2.380955631371226e-06 -1.0 2.1648918040467363e-05 -1.0 0.0833931797381589 -1.0 -1.0 -1.0 0.08285899874014539 -1.0 2.762955748645507e-05 -1.0 5.1161740516205715e-05 -1.0 0.08095280568305474 -1.0 -1.0 -1.0 0.08636569925236791 -1.0 3.452537179033895e-05 -1.0 3.5981031766416143e-06 -1.0 0.001570829313580841 -1.0 0.01621821782442112 -1.0 0.0015447823995470042 -1.0 7.989579285134734e-06 -1.0 -1.0 -1.0 2.594951085377588e-06 -1.0 4.323138781337963e-05 -1.0 6.948292975998466e-07 -1.0 -1.0 -1.0 -1.0 -1.0 2.4469456008493614e-09 -1.0 9.041149893307806e-07 -1.0 2.2060992580622125e-11 -1.0 -1.0 -1.0 1.681305446488884e-11 -1.0 7.091263906557291e-05 -1.0 0.00027464244062887683 -1.0 1.28584557092134e-05 -1.0 2.0295213621716706e-09 -1.0 1.2911827198500372e-08 -1.0 0.0009127050763987511 -1.0 0.007385384405891666 -1.0 0.0010019432401508087 -1.0 1.080711295768551e-05 -1.0 8.020767115181574e-07 -1.0 0.0009252532821720597 -1.0 0.007188410694198128 -1.0 0.0007245451610090619 -1.0 2.5157355790201773e-07 -1.0 1.4825219199133353e-12 -1.0 4.756872959630257e-05 -1.0 0.0002668579837809081 -1.0 2.8226276944532696e-05 -1.0 5.182269672663266e-10 -1.0 -1.0 -1.0 2.936268747135548e-14 -1.0 2.5447281241730366e-07 -1.0 3.89841667138598e-08 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 2.639268952045579e-10 -1.0 3.7051518220083886e-06 -1.0 1.1333627604823673e-09 -1.0 -1.0 -1.0 -1.0 -1.0 1.2296747260635405e-06 -1.0 7.962899789920809e-06 -1.0 8.211982683649991e-09 -1.0 -1.0 -1.0 -1.0 -1.0 2.5639656866250453e-06 -1.0 2.2487617828938326e-05 -1.0 3.2799045883372075e-06 -1.0 4.076762579823379e-17 -1.0 1.5476769237571295e-14 -1.0 1.0305672698745657e-11 -1.0 1.957092814065688e-05 -1.0 3.8523247550378815e-12 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 2.7697233171399416e-13 -1.0 -1.0 -1.0 -1.0 -1.0</lower_ww_bounds>
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0.01178616843705956 -10.0 0.04441328419270369 -10.0 0.008919466185572301 -10.0 5.272021975060514e-05 -10.0 0.0016079768514218546 -10.0 0.11128424196387618 -10.0 0.6433819434431045 -10.0 0.10955637485319249 -10.0 0.0008370896020975689 -10.0 0.0022234471595443312 -10.0 0.1102706140784316 -10.0 0.6684484191345574 -10.0 0.11113024641218336 -10.0 0.0012558879474108074 -10.0 0.00010044390728858278 -10.0 0.00808100901275472 -10.0 0.05384625561469684 -10.0 0.01146714816952061 -10.0 3.574060778841494e-05 -10.0 2.2512327083641864e-09 -10.0 0.0006717625881826168 -10.0 0.0006949587369646498 -10.0 0.0002909334597507523 -10.0 -10.0 -10.0 -10.0 -10.0 8.582011716859219e-14 -10.0 8.304845400451923e-07 -10.0 4.5757238372058235e-10 -10.0 -10.0 -10.0 1.3116807133030034e-14 -10.0 0.00026866894852481903 -10.0 0.00043166052799897375 -10.0 6.401055165502385e-05 -10.0 5.726152701849347e-15 -10.0 8.541606727879512e-06 -10.0 0.0020293965453828133 -10.0 0.004299195570308035 -10.0 0.0008544159282151438 -10.0 1.0995322456277382e-05 -10.0 1.289814511028015e-05 -10.0 0.0011042047857307254 -10.0 0.006870228272606287 -10.0 0.0013998074566164962 -10.0 4.279460329782799e-08 -10.0 1.2907171695294846e-12 -10.0 0.00019257248009114364 -10.0 0.0009332371598523186 -10.0 0.000131063355181338 -10.0 6.574706543646946e-15 -10.0 -10.0 -10.0 3.8270536025799805e-14 -10.0 2.101790478097517e-08 -10.0 9.904481358675478e-12 -10.0 -10.0</upper_ww_bounds>
     <survival_ratio>3</survival_ratio>
+    <max_lower_bound_ratio>1.0</max_lower_bound_ratio>
     <max_split>10</max_split>
     <weight_cutoff>1e-38</weight_cutoff>
   </weight_windows>
diff --git a/tests/regression_tests/weightwindows/test.py b/tests/regression_tests/weightwindows/test.py
index e9e64b0466..a2708ac4bf 100644
--- a/tests/regression_tests/weightwindows/test.py
+++ b/tests/regression_tests/weightwindows/test.py
@@ -93,14 +93,14 @@ def model():
                                 None,
                                 10.0,
                                 e_bnds,
-                                max_lb_ratio=5.0)
+                                max_lb_ratio=1.0)
 
     ww_p = openmc.WeightWindows(ww_mesh,
                                 ww_p_lower_bnds,
                                 None,
                                 10.0,
                                 e_bnds,
-                                max_lb_ratio=5.0)
+                                max_lb_ratio=1.0)
 
     model.settings.weight_windows = [ww_n, ww_p]
 

From c6d4f1a5edbb6f2837e4dab27a58846182cfb000 Mon Sep 17 00:00:00 2001
From: Patrick Shriwise <pshriwise@gmail.com>
Date: Wed, 26 Jan 2022 13:41:35 -0600
Subject: [PATCH 04/11] Updating some data member formats.

---
 include/openmc/particle_data.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/openmc/particle_data.h b/include/openmc/particle_data.h
index 128685fef2..727be0ec0e 100644
--- a/include/openmc/particle_data.h
+++ b/include/openmc/particle_data.h
@@ -310,8 +310,8 @@ private:
 
   // Weight window information
   int n_split_ {0}; // Number of times this particle has been split
-  double ww_factor_ {
-    0.0}; // Particle-specific factor for on-the-fly weight window adjustment
+  // Particle-specific factor for on-the-fly weight window adjustment
+  double ww_factor_ {0.0};
 
 // DagMC state variables
 #ifdef DAGMC

From 70d16656431932554977417b34bd47f6e835757b Mon Sep 17 00:00:00 2001
From: Patrick Shriwise <pshriwise@gmail.com>
Date: Wed, 26 Jan 2022 15:22:51 -0600
Subject: [PATCH 05/11] Apply suggestions from code review

Co-authored-by: Ethan Peterson <ethan.peterson@mit.edu>
---
 openmc/weight_windows.py | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/openmc/weight_windows.py b/openmc/weight_windows.py
index df46e318c0..f19dfcadc8 100644
--- a/openmc/weight_windows.py
+++ b/openmc/weight_windows.py
@@ -49,8 +49,9 @@ class WeightWindows(IDManagerMixin):
         Ratio of the survival weight to the lower weight window bound for
         rouletting
     max_lower_bound_ratio: float
-        Maximum allowed ratio of a weight window's lower bound to particle weight.
-        A factor will be applied to raise the weight window to the particle's weight
+        Maximum allowed ratio of a particle's weight to the weight window's
+	lower bound. A factor will be applied to raise the weight window to
+	be lower than the particle's weight by a factor of max_lower_bound_ratio
         during transport if exceeded. Only applied if > 1.0. (Default: 1.0).
     max_split : int
         Maximum allowable number of particles when splitting
@@ -81,8 +82,8 @@ class WeightWindows(IDManagerMixin):
         Ratio of the survival weight to the lower weight window bound for
         rouletting
     max_lower_bound_ratio: float
-        Maximum allowed ratio of a weight window's lower bound to particle
-        weight. (Default: 1.0)
+        Maximum allowed ratio of a particle's weight to the weight window's
+        lower bound. (Default: 1.0)
     max_split : int
         Maximum allowable number of particles when splitting
     weight_cutoff : float

From aefcb4c4aaea03f56fbf867d9c67924f366fad18 Mon Sep 17 00:00:00 2001
From: Patrick Shriwise <pshriwise@gmail.com>
Date: Tue, 1 Feb 2022 12:17:38 -0600
Subject: [PATCH 06/11] Addressing comments from @paulromano

---
 docs/source/io_formats/settings.rst |  6 ++++++
 include/openmc/particle_data.h      |  3 +--
 openmc/weight_windows.py            | 31 +++++++++++++++++++----------
 3 files changed, 28 insertions(+), 12 deletions(-)

diff --git a/docs/source/io_formats/settings.rst b/docs/source/io_formats/settings.rst
index ead440ae9b..7f58ee3ab3 100644
--- a/docs/source/io_formats/settings.rst
+++ b/docs/source/io_formats/settings.rst
@@ -1050,6 +1050,12 @@ sub-elements/attributes:
 
     *Default*: 3.0
 
+  :max_lower_bound_ratio:
+    Maximum allowed ratio of a particle's weight to the weight window's lower
+    bound. A factor will be applied to raise the weight window to be lower than
+	  the particle's weight by a factor of max_lower_bound_ratio during transport
+    if exceeded.
+
   :max_split:
     Maximum allowable number of particles when splitting
 
diff --git a/include/openmc/particle_data.h b/include/openmc/particle_data.h
index 727be0ec0e..b3a31ff42d 100644
--- a/include/openmc/particle_data.h
+++ b/include/openmc/particle_data.h
@@ -310,8 +310,7 @@ private:
 
   // Weight window information
   int n_split_ {0}; // Number of times this particle has been split
-  // Particle-specific factor for on-the-fly weight window adjustment
-  double ww_factor_ {0.0};
+  double ww_factor_ {0.0}; // Particle-specific factor for on-the-fly weight window adjustment
 
 // DagMC state variables
 #ifdef DAGMC
diff --git a/openmc/weight_windows.py b/openmc/weight_windows.py
index f19dfcadc8..118f58fca6 100644
--- a/openmc/weight_windows.py
+++ b/openmc/weight_windows.py
@@ -48,11 +48,11 @@ class WeightWindows(IDManagerMixin):
     survival_ratio : float
         Ratio of the survival weight to the lower weight window bound for
         rouletting
-    max_lower_bound_ratio: float
+    max_lower_bound_ratio : float
         Maximum allowed ratio of a particle's weight to the weight window's
 	lower bound. A factor will be applied to raise the weight window to
 	be lower than the particle's weight by a factor of max_lower_bound_ratio
-        during transport if exceeded. Only applied if > 1.0. (Default: 1.0).
+        during transport if exceeded.
     max_split : int
         Maximum allowable number of particles when splitting
     weight_cutoff : float
@@ -99,7 +99,8 @@ class WeightWindows(IDManagerMixin):
 
     def __init__(self, mesh, lower_ww_bounds, upper_ww_bounds=None,
                  upper_bound_ratio=None, energy_bins=None, particle_type='neutron',
-                 survival_ratio=3, max_lb_ratio=1.0, max_split=10, weight_cutoff=1.e-38, id=None):
+                 survival_ratio=3, max_lower_bound_ratio=None, max_split=10,
+                 weight_cutoff=1.e-38, id=None):
         self.mesh = mesh
         self.id = id
         self.particle_type = particle_type
@@ -129,7 +130,8 @@ class WeightWindows(IDManagerMixin):
                              'do not match')
 
         self.survival_ratio = survival_ratio
-        self.max_lower_bound_ratio = max_lb_ratio
+        if self.max_lower_bound_ratio is not None:
+            self.max_lower_bound_ratio = max_lower_bound_ratio
         self.max_split = max_split
         self.weight_cutoff = weight_cutoff
 
@@ -260,8 +262,9 @@ class WeightWindows(IDManagerMixin):
         subelement = ET.SubElement(element, 'survival_ratio')
         subelement.text = str(self.survival_ratio)
 
-        subelement = ET.SubElement(element, 'max_lower_bound_ratio')
-        subelement.text = str(self.max_lower_bound_ratio)
+        if self.max_lower_bound_ratio is not None:
+            subelement = ET.SubElement(element, 'max_lower_bound_ratio')
+            subelement.text = str(self.max_lower_bound_ratio)
 
         subelement = ET.SubElement(element, 'max_split')
         subelement.text = str(self.max_split)
@@ -300,7 +303,11 @@ class WeightWindows(IDManagerMixin):
         ebins = [float(b) for b in get_text(elem, 'energy_bins').split()]
         particle_type = get_text(elem, 'particle_type')
         survival_ratio = float(get_text(elem, 'survival_ratio'))
-        max_lb_ratio = float(get_text(elem, 'max_lower_bound_ratio'))
+
+        max_lower_bound_ratio = None
+        if get_text(elem, 'max_lower_bound_ratio'):
+            max_lower_bound_ratio = float(get_text(elem, 'max_lower_bound_ratio'))
+
         max_split = int(get_text(elem, 'max_split'))
         weight_cutoff = float(get_text(elem, 'weight_cutoff'))
         id = int(get_text(elem, 'id'))
@@ -312,7 +319,7 @@ class WeightWindows(IDManagerMixin):
             energy_bins=ebins,
             particle_type=particle_type,
             survival_ratio=survival_ratio,
-            max_lb_ratio=max_lb_ratio,
+            max_lower_bound_ratio=max_lower_bound_ratio,
             max_split=max_split,
             weight_cutoff=weight_cutoff,
             id=id
@@ -342,7 +349,11 @@ class WeightWindows(IDManagerMixin):
         lower_ww_bounds = group['lower_ww_bounds'][()]
         upper_ww_bounds = group['upper_ww_bounds'][()]
         survival_ratio = group['survival_ratio'][()]
-        max_lb_ratio = group['max_lower_bound_ratio'][()]
+
+        max_lower_bound_ratio = None
+        if group.get('max_lower_bound_ratio') is not None:
+            max_lower_bound_ratio = group['max_lower_bound_ratio'][()]
+
         max_split = group['max_split'][()]
         weight_cutoff = group['weight_cutoff'][()]
 
@@ -353,7 +364,7 @@ class WeightWindows(IDManagerMixin):
             energy_bins=ebins,
             particle_type=ptype,
             survival_ratio=survival_ratio,
-            max_lb_ratio=max_lb_ratio,
+            max_lower_bound_ratio=max_lower_bound_ratio,
             max_split=max_split,
             weight_cutoff=weight_cutoff,
             id=id

From e27ba6cd1eef60e47d87a3b5134d345b3d67014e Mon Sep 17 00:00:00 2001
From: Patrick Shriwise <pshriwise@gmail.com>
Date: Tue, 1 Feb 2022 13:11:18 -0600
Subject: [PATCH 07/11] Adding max_lower_bound_ratio internal attribute in WW
 constructor

---
 openmc/weight_windows.py | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/openmc/weight_windows.py b/openmc/weight_windows.py
index 118f58fca6..ae4a1e6317 100644
--- a/openmc/weight_windows.py
+++ b/openmc/weight_windows.py
@@ -130,8 +130,11 @@ class WeightWindows(IDManagerMixin):
                              'do not match')
 
         self.survival_ratio = survival_ratio
-        if self.max_lower_bound_ratio is not None:
+
+        self._max_lower_bound_ratio = None
+        if max_lower_bound_ratio is not None:
             self.max_lower_bound_ratio = max_lower_bound_ratio
+
         self.max_split = max_split
         self.weight_cutoff = weight_cutoff
 

From 1e8ede8fef725975b99a92897319c925b1e5166e Mon Sep 17 00:00:00 2001
From: Patrick Shriwise <pshriwise@gmail.com>
Date: Tue, 1 Feb 2022 15:39:39 -0600
Subject: [PATCH 08/11] correcting keyword arg for max lower bound ratio

---
 tests/regression_tests/weightwindows/test.py | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/tests/regression_tests/weightwindows/test.py b/tests/regression_tests/weightwindows/test.py
index a2708ac4bf..b1656e0e59 100644
--- a/tests/regression_tests/weightwindows/test.py
+++ b/tests/regression_tests/weightwindows/test.py
@@ -93,14 +93,14 @@ def model():
                                 None,
                                 10.0,
                                 e_bnds,
-                                max_lb_ratio=1.0)
+                                max_lower_bound_ratio=1.0)
 
     ww_p = openmc.WeightWindows(ww_mesh,
                                 ww_p_lower_bnds,
                                 None,
                                 10.0,
                                 e_bnds,
-                                max_lb_ratio=1.0)
+                                max_lower_bound_ratio=1.0)
 
     model.settings.weight_windows = [ww_n, ww_p]
 

From 5d5774b5bf90f75da7e31342712a5ec03a8ea4b6 Mon Sep 17 00:00:00 2001
From: Patrick Shriwise <pshriwise@gmail.com>
Date: Wed, 2 Feb 2022 09:58:51 -0600
Subject: [PATCH 09/11] correcting indentation. requiring max_lower_bound_ratio
 to be > 1. updating test.

---
 openmc/weight_windows.py                              | 6 +++---
 tests/regression_tests/weightwindows/inputs_true.dat  | 4 ++--
 tests/regression_tests/weightwindows/results_true.dat | 2 +-
 tests/regression_tests/weightwindows/test.py          | 4 ++--
 4 files changed, 8 insertions(+), 8 deletions(-)

diff --git a/openmc/weight_windows.py b/openmc/weight_windows.py
index ae4a1e6317..5d77dada46 100644
--- a/openmc/weight_windows.py
+++ b/openmc/weight_windows.py
@@ -50,8 +50,8 @@ class WeightWindows(IDManagerMixin):
         rouletting
     max_lower_bound_ratio : float
         Maximum allowed ratio of a particle's weight to the weight window's
-	lower bound. A factor will be applied to raise the weight window to
-	be lower than the particle's weight by a factor of max_lower_bound_ratio
+        lower bound. A factor will be applied to raise the weight window to be lower
+        than the particle's weight by a factor of max_lower_bound_ratio
         during transport if exceeded.
     max_split : int
         Maximum allowable number of particles when splitting
@@ -213,7 +213,7 @@ class WeightWindows(IDManagerMixin):
     @max_lower_bound_ratio.setter
     def max_lower_bound_ratio(self, val):
         cv.check_type('Maximum lower bound ratio', val, Real)
-        cv.check_greater_than('Maximum lower bound ratio', val, 1.0, True)
+        cv.check_greater_than('Maximum lower bound ratio', val, 1.0)
         self._max_lower_bound_ratio = val
 
     @property
diff --git a/tests/regression_tests/weightwindows/inputs_true.dat b/tests/regression_tests/weightwindows/inputs_true.dat
index c7720d1dfe..13641bf7aa 100644
--- a/tests/regression_tests/weightwindows/inputs_true.dat
+++ b/tests/regression_tests/weightwindows/inputs_true.dat
@@ -51,7 +51,7 @@
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-1.0 0.00016079768514218546 -1.0 0.011128424196387618 -1.0 0.06433819434431046 -1.0 0.010955637485319249 -1.0 8.370896020975689e-05 -1.0 0.00022234471595443313 -1.0 0.01102706140784316 -1.0 0.06684484191345574 -1.0 0.011113024641218336 -1.0 0.00012558879474108074 -1.0 1.0044390728858277e-05 -1.0 0.000808100901275472 -1.0 0.005384625561469684 -1.0 0.001146714816952061 -1.0 3.5740607788414942e-06 -1.0 2.2512327083641865e-10 -1.0 6.717625881826167e-05 -1.0 6.949587369646497e-05 -1.0 2.9093345975075226e-05 -1.0 -1.0 -1.0 -1.0 -1.0 8.582011716859219e-15 -1.0 8.304845400451923e-08 -1.0 4.5757238372058234e-11 -1.0 -1.0 -1.0 1.3116807133030035e-15 -1.0 2.6866894852481906e-05 -1.0 4.3166052799897375e-05 -1.0 6.401055165502384e-06 -1.0 5.726152701849347e-16 -1.0 8.541606727879512e-07 -1.0 0.0002029396545382813 -1.0 0.0004299195570308035 -1.0 8.544159282151438e-05 -1.0 1.0995322456277382e-06 -1.0 1.289814511028015e-06 -1.0 0.00011042047857307253 -1.0 0.0006870228272606287 -1.0 0.0001399807456616496 -1.0 4.279460329782799e-09 -1.0 1.2907171695294846e-13 -1.0 1.9257248009114363e-05 -1.0 9.332371598523186e-05 -1.0 1.3106335518133802e-05 -1.0 6.574706543646946e-16 -1.0 -1.0 -1.0 3.8270536025799805e-15 -1.0 2.101790478097517e-09 -1.0 9.904481358675478e-13 -1.0 -1.0</lower_ww_bounds>
     <upper_ww_bounds>-10.0 -10.0 -10.0 5.447493368067179e-09 -10.0 4.0404626147344005e-12 -10.0 9.382800796426367e-16 -10.0 -10.0 -10.0 5.752120832330886e-10 -10.0 0.0003520674728719278 -10.0 0.0008760419353035269 -10.0 0.0003136829920007009 -10.0 -10.0 -10.0 4.233410073487772e-05 -10.0 0.0025519632243117644 -10.0 0.004777967757842694 -10.0 0.0035983547999740664 -10.0 4.625859206051786e-06 -10.0 4.587143797469485e-07 -10.0 0.0012918433677593976 -10.0 0.0046958956846326495 -10.0 0.0008862760180332873 -10.0 3.279091474401153e-05 -10.0 1.151457911727229e-09 -10.0 0.00022950716170641464 -10.0 0.0006922962992428139 -10.0 0.00021606550732170693 -10.0 2.6178780551069492e-15 -10.0 -10.0 -10.0 -10.0 -10.0 1.722191123879782e-07 -10.0 -10.0 -10.0 -10.0 -10.0 2.123498061876322e-12 -10.0 0.00014628847959717226 -10.0 0.0002777791741571174 -10.0 9.53380658669839e-05 -10.0 1.078687446476881e-13 -10.0 3.0571050855707086e-05 -10.0 0.008231482263927564 -10.0 0.03881102561422506 -10.0 0.012297303081805393 -10.0 0.00046842350882367664 -10.0 0.0016757997590048828 -10.0 0.11724714376848187 -10.0 0.6827568359008944 -10.0 0.11440381012864086 -10.0 0.003782322077390033 -10.0 0.0005098747893974248 -10.0 0.1208261392046878 -10.0 0.6428782790571179 -10.0 0.11294708566804167 -10.0 0.0008168081568291806 -10.0 0.000234750482787598 -10.0 0.008787411098754914 -10.0 0.0515132921322595 -10.0 0.011028020390507681 -10.0 1.0882011018684853e-05 -10.0 1.0588257623722672e-06 -10.0 0.00027044817176109554 -10.0 0.000262112392467961 -10.0 2.0707648529746893e-05 -10.0 -10.0 -10.0 1.3806577785135176e-05 -10.0 0.0010077009726691265 -10.0 0.004313097771132342 -10.0 0.001335621430311088 -10.0 3.792209399652396e-05 -10.0 0.0022057970761624767 -10.0 0.1884234453311008 -10.0 1.1510120013926417 -10.0 0.1825795827020865 -10.0 0.0020848309220162036 -10.0 0.013332554437480326 -10.0 4.931708164307665 -10.0 -10.0 -10.0 4.899952354672477 -10.0 0.014957033281168012 -10.0 0.01550036616828061 -10.0 4.897735047310072 -10.0 -10.0 -10.0 5.0 -10.0 0.011076796092102187 -10.0 0.002792896409558745 -10.0 0.17218062957363772 -10.0 1.1891251244906933 -10.0 0.19040707071165303 -10.0 0.0017329528628152924 -10.0 4.670673837764874e-07 -10.0 0.0012637534695610975 -10.0 0.0027052054298385255 -10.0 0.001497958445994315 -10.0 3.2478301680021856e-07 -10.0 7.239087311622819e-07 -10.0 0.004669378458601891 -10.0 0.01537590300149361 -10.0 0.002368681880710174 -10.0 8.61602729455546e-06 -10.0 0.002362355168421952 -10.0 0.5708498338352204 -10.0 4.237271368372502 -10.0 0.5490281661912204 -10.0 0.0030002326388383245 -10.0 0.023295401596005315 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 0.027847329307350423 -10.0 0.03432919201293737 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 0.026729237733633893 -10.0 0.0031821087819554777 -10.0 0.5639071613735579 -10.0 4.346984808472855 -10.0 0.5500108683629957 -10.0 0.0024175998138228046 -10.0 2.774067049392004e-06 -10.0 0.0032576461348374514 -10.0 0.012648333883989995 -10.0 0.002981891705231209 -10.0 4.986698316296507e-05 -10.0 3.419181071650816e-05 -10.0 0.0010814452764860557 -10.0 0.006934466049804711 -10.0 0.0025106741008066318 -10.0 5.515522196186723e-05 -10.0 0.0014242053195529865 -10.0 0.18541112907970003 -10.0 1.2364069326350118 -10.0 0.17602971074583987 -10.0 0.0021305826462238594 -10.0 0.015016500045374082 -10.0 4.903495510314233 -10.0 -10.0 -10.0 4.952432605379829 -10.0 0.01769327994909422 -10.0 0.00930449789620512 -10.0 4.873897581604287 -10.0 -10.0 -10.0 4.981497862127628 -10.0 0.011273340972061077 -10.0 0.0014126153603265096 -10.0 0.1729816946709781 -10.0 1.1819165666519222 -10.0 0.1780262484403569 -10.0 0.001319677450153934 -10.0 2.210707325798542e-06 -10.0 0.0017287873387910357 -10.0 0.0051970818427099184 -10.0 0.0009256616392487858 -10.0 1.0325207105027347e-07 -10.0 2.027381220364608e-15 -10.0 0.00012257360246712473 -10.0 0.00038561696337086434 -10.0 0.00018064550990294753 -10.0 7.6915277818290365e-16 -10.0 0.00012980629616179268 -10.0 0.01178616843705956 -10.0 0.04441328419270369 -10.0 0.008919466185572301 -10.0 5.272021975060514e-05 -10.0 0.0016079768514218546 -10.0 0.11128424196387618 -10.0 0.6433819434431045 -10.0 0.10955637485319249 -10.0 0.0008370896020975689 -10.0 0.0022234471595443312 -10.0 0.1102706140784316 -10.0 0.6684484191345574 -10.0 0.11113024641218336 -10.0 0.0012558879474108074 -10.0 0.00010044390728858278 -10.0 0.00808100901275472 -10.0 0.05384625561469684 -10.0 0.01146714816952061 -10.0 3.574060778841494e-05 -10.0 2.2512327083641864e-09 -10.0 0.0006717625881826168 -10.0 0.0006949587369646498 -10.0 0.0002909334597507523 -10.0 -10.0 -10.0 -10.0 -10.0 8.582011716859219e-14 -10.0 8.304845400451923e-07 -10.0 4.5757238372058235e-10 -10.0 -10.0 -10.0 1.3116807133030034e-14 -10.0 0.00026866894852481903 -10.0 0.00043166052799897375 -10.0 6.401055165502385e-05 -10.0 5.726152701849347e-15 -10.0 8.541606727879512e-06 -10.0 0.0020293965453828133 -10.0 0.004299195570308035 -10.0 0.0008544159282151438 -10.0 1.0995322456277382e-05 -10.0 1.289814511028015e-05 -10.0 0.0011042047857307254 -10.0 0.006870228272606287 -10.0 0.0013998074566164962 -10.0 4.279460329782799e-08 -10.0 1.2907171695294846e-12 -10.0 0.00019257248009114364 -10.0 0.0009332371598523186 -10.0 0.000131063355181338 -10.0 6.574706543646946e-15 -10.0 -10.0 -10.0 3.8270536025799805e-14 -10.0 2.101790478097517e-08 -10.0 9.904481358675478e-12 -10.0 -10.0</upper_ww_bounds>
     <survival_ratio>3</survival_ratio>
-    <max_lower_bound_ratio>1.0</max_lower_bound_ratio>
+    <max_lower_bound_ratio>1.5</max_lower_bound_ratio>
     <max_split>10</max_split>
     <weight_cutoff>1e-38</weight_cutoff>
   </weight_windows>
diff --git a/tests/regression_tests/weightwindows/results_true.dat b/tests/regression_tests/weightwindows/results_true.dat
index 5e8a41faf9..971401995b 100644
--- a/tests/regression_tests/weightwindows/results_true.dat
+++ b/tests/regression_tests/weightwindows/results_true.dat
@@ -1 +1 @@
-c77af605fb23bc3f03bb304b82cc2d0d9efe7826167f43202fa0ecba17e6a77fc1a91bf9f4b23f175ae98c04c69067bffb96d7d68e30a126f5fa7c0080ea8367
\ No newline at end of file
+f10c722e27d1f0a69f700bc72c4b7751f375dc0a74e049c9abb4b261d2265155c0e516e7e38f9751b83a24eac07e944e51740d398b720524cf8cd6bf0b8c51fc
\ No newline at end of file
diff --git a/tests/regression_tests/weightwindows/test.py b/tests/regression_tests/weightwindows/test.py
index b1656e0e59..3d3cf9d7ae 100644
--- a/tests/regression_tests/weightwindows/test.py
+++ b/tests/regression_tests/weightwindows/test.py
@@ -93,14 +93,14 @@ def model():
                                 None,
                                 10.0,
                                 e_bnds,
-                                max_lower_bound_ratio=1.0)
+                                max_lower_bound_ratio=1.5)
 
     ww_p = openmc.WeightWindows(ww_mesh,
                                 ww_p_lower_bnds,
                                 None,
                                 10.0,
                                 e_bnds,
-                                max_lower_bound_ratio=1.0)
+                                max_lower_bound_ratio=1.5)
 
     model.settings.weight_windows = [ww_n, ww_p]
 

From c0f1fb85e745731c618bb893cf02d243ffd054eb Mon Sep 17 00:00:00 2001
From: Patrick Shriwise <pshriwise@gmail.com>
Date: Wed, 2 Feb 2022 12:08:52 -0600
Subject: [PATCH 10/11] Fixing indentation.

---
 docs/source/io_formats/settings.rst | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/docs/source/io_formats/settings.rst b/docs/source/io_formats/settings.rst
index 7f58ee3ab3..d1d9e69fac 100644
--- a/docs/source/io_formats/settings.rst
+++ b/docs/source/io_formats/settings.rst
@@ -1053,7 +1053,7 @@ sub-elements/attributes:
   :max_lower_bound_ratio:
     Maximum allowed ratio of a particle's weight to the weight window's lower
     bound. A factor will be applied to raise the weight window to be lower than
-	  the particle's weight by a factor of max_lower_bound_ratio during transport
+    the particle's weight by a factor of max_lower_bound_ratio during transport
     if exceeded.
 
   :max_split:

From 86ac407544106e3ff3c46960e44271bde43eede7 Mon Sep 17 00:00:00 2001
From: Patrick Shriwise <pshriwise@gmail.com>
Date: Wed, 2 Feb 2022 17:46:43 -0600
Subject: [PATCH 11/11] Reset ww ratio when initializing particle history

---
 src/simulation.cpp | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/src/simulation.cpp b/src/simulation.cpp
index 772c909505..b7f5b58f16 100644
--- a/src/simulation.cpp
+++ b/src/simulation.cpp
@@ -490,6 +490,9 @@ void initialize_history(Particle& p, int64_t index_source)
   // Reset split counter
   p.n_split() = 0;
 
+  // Reset weight window ratio
+  p.ww_factor() = 0.0;
+
   // set random number seed
   int64_t particle_seed =
     (simulation::total_gen + overall_generation() - 1) * settings::n_particles +