From 102bfd47aea00e9570344c334ba553e82c14e2e1 Mon Sep 17 00:00:00 2001
From: Paul Romano <paul.k.romano@gmail.com>
Date: Tue, 8 Dec 2020 07:24:51 -0600
Subject: [PATCH] Fix use of 'nearest' temperature with S(a,b) data. Closes
 #1704

---
 src/thermal.cpp | 24 ++++++++++++------------
 1 file changed, 12 insertions(+), 12 deletions(-)

diff --git a/src/thermal.cpp b/src/thermal.cpp
index f8e9b280be..4bb0487188 100644
--- a/src/thermal.cpp
+++ b/src/thermal.cpp
@@ -151,15 +151,9 @@ ThermalScattering::calculate_xs(double E, double sqrtkT, int* i_temp,
 {
   // Determine temperature for S(a,b) table
   double kT = sqrtkT*sqrtkT;
-  int i;
-  if (settings::temperature_method == TemperatureMethod::NEAREST) {
-    // If using nearest temperature, do linear search on temperature
-    for (i = 0; i < kTs_.size(); ++i) {
-      if (std::abs(kTs_[i] - kT) < K_BOLTZMANN*settings::temperature_tolerance) {
-        break;
-      }
-    }
-  } else {
+  int i = 0;
+
+  if (kTs_.size() > 1) {
     // Find temperatures that bound the actual temperature
     for (i = 0; i < kTs_.size() - 1; ++i) {
       if (kTs_[i] <= kT && kT < kTs_[i+1]) {
@@ -167,9 +161,15 @@ ThermalScattering::calculate_xs(double E, double sqrtkT, int* i_temp,
       }
     }
 
-    // Randomly sample between temperature i and i+1
-    double f = (kT - kTs_[i]) / (kTs_[i+1] - kTs_[i]);
-    if (f > prn(seed)) ++i;
+    if (settings::temperature_method == TemperatureMethod::NEAREST) {
+      // Pick closer of two bounding temperatures
+      if (kT - kTs_[i] > kTs_[i+1] - kT) ++i;
+
+    } else {
+      // Randomly sample between temperature i and i+1
+      double f = (kT - kTs_[i]) / (kTs_[i+1] - kTs_[i]);
+      if (f > prn(seed)) ++i;
+    }
   }
 
   // Set temperature index