diff --git a/src/constants.F90 b/src/constants.F90
index 89490eb183..67e7877467 100644
--- a/src/constants.F90
+++ b/src/constants.F90
@@ -312,6 +312,12 @@ module constants
   integer, parameter :: ERROR_INT  = -huge(0)
   real(8), parameter :: ERROR_REAL = -huge(0.0_8) * 0.917826354_8
 
+  ! Energy grid methods
+  integer, parameter :: &
+       GRID_NUCLIDE  = 1, & ! non-unionized energy grid (MCNP)
+       GRID_UNION    = 2, & ! union grid with pointers
+       GRID_LETHARGY = 3    ! lethargy mapping (MC21)
+
   ! Source types
   integer, parameter ::   &
        SRC_BOX     = 1, & ! Source in a rectangular prism
diff --git a/src/cross_section.F90 b/src/cross_section.F90
index 966ff1cd94..b43283e037 100644
--- a/src/cross_section.F90
+++ b/src/cross_section.F90
@@ -37,7 +37,7 @@ contains
     mat => materials(p % material)
 
     ! Find energy index on unionized grid
-    call find_energy_index()
+    if (grid_method == GRID_UNION) call find_energy_index()
 
     ! Check if there's an S(a,b) table for this material
     if (mat % has_sab_table) then
@@ -107,11 +107,28 @@ contains
     ! Set pointer to nuclide
     nuc => nuclides(index_nuclide)
 
-    ! TODO: If not using unionized energy grid, we need to find the index on the
-    ! nuclide energy grid using lethargy mapping or whatever other technique
+    ! Determine index on nuclide energy grid
+    select case (grid_method)
+    case (GRID_UNION)
+       ! If we're using the unionized grid with pointers, finding the index on
+       ! the nuclide energy grid is as simple as looking up the pointer
 
-    ! get index on nuclide energy grid
-    IE = nuc % grid_index(p % IE)
+       IE = nuc % grid_index(p % IE)
+
+    case (GRID_NUCLIDE)
+       ! If we're not using the unionized grid, we have to do a binary search on
+       ! the nuclide energy grid in order to determine which points to
+       ! interpolate between
+
+       if (p % E < nuc % energy(1)) then
+          IE = 1
+       elseif (p % E > nuc % energy(nuc % n_grid)) then
+          IE = nuc % n_grid - 1
+       else
+          IE = binary_search(nuc % energy, nuc % n_grid, p % E)
+       end if
+
+    end select
 
     ! check for rare case where two energy points are the same
     if (nuc % energy(IE) == nuc % energy(IE+1)) IE = IE + 1
diff --git a/src/global.F90 b/src/global.F90
index d7edf18215..c64ebedc9d 100644
--- a/src/global.F90
+++ b/src/global.F90
@@ -82,7 +82,8 @@ module global
   type(DictionaryCI), pointer :: xs_listing_dict => null()
 
   ! Unionized energy grid
-  integer              :: n_grid    ! number of points on unionized grid
+  integer :: grid_method ! how to treat the energy grid
+  integer :: n_grid      ! number of points on unionized grid
   real(8), allocatable :: e_grid(:) ! energies on unionized grid
 
   ! Unreoslved resonance probablity tables
diff --git a/src/initialize.F90 b/src/initialize.F90
index 697273533e..3d3719b48d 100644
--- a/src/initialize.F90
+++ b/src/initialize.F90
@@ -106,9 +106,11 @@ contains
        call timer_stop(time_read_xs)
 
        ! Construct unionized energy grid from cross-sections
-       call timer_start(time_unionize)
-       call unionized_grid()
-       call timer_stop(time_unionize)
+       if (grid_method == GRID_UNION) then
+          call timer_start(time_unionize)
+          call unionized_grid()
+          call timer_stop(time_unionize)
+       end if
 
        ! Create tally map
        call create_tally_map()
diff --git a/src/input_xml.F90 b/src/input_xml.F90
index 29c72309a9..c1257855e1 100644
--- a/src/input_xml.F90
+++ b/src/input_xml.F90
@@ -67,6 +67,7 @@ contains
     ! Initialize XML scalar variables
     cross_sections_ = ""
     verbosity_ = 0
+    energy_grid_ = "union"
 
     ! Parse settings.xml file
     call read_xml_file_settings_t(filename)
@@ -111,6 +112,20 @@ contains
        call fatal_error()
     end if
 
+    ! Energy grid methods
+    select case (energy_grid_)
+    case ('nuclide')
+       grid_method = GRID_NUCLIDE
+    case ('union')
+       grid_method = GRID_UNION
+    case ('lethargy')
+       message = "Lethargy mapped energy grid not yet supported."
+       call fatal_error()
+    case default
+       message = "Unknown energy grid method: " // energy_grid_
+       call fatal_error()
+    end select
+
     ! Verbosity
     if (verbosity_ > 0) verbosity = verbosity_
 
diff --git a/src/xml-fortran/templates/settings_t.xml b/src/xml-fortran/templates/settings_t.xml
index 69388420ca..4b450f5724 100644
--- a/src/xml-fortran/templates/settings_t.xml
+++ b/src/xml-fortran/templates/settings_t.xml
@@ -27,13 +27,14 @@
   </typedef>
 
   <variable name="criticality" type="criticality_xml" />
-  <variable name="verbosity_" tag="verbosity" type="integer" />
+  <variable name="cross_sections_" tag="cross_sections" type="word" length="255" />
+  <variable name="cutoff_" tag="cutoff" type="cutoff_xml" dimension="1" />
+  <variable name="energy_grid_" tag="energy_grid" type="word" length="7" />
+  <variable name="entropy_" tag="entropy" type="entropy_xml" dimension="1" />
+  <variable name="ptables_" tag="ptables" type="word" length="3" />
   <variable name="source_" tag="source" type="source_xml" />
   <variable name="survival_" tag="survival_biasing" type="word" length="3" />
-  <variable name="ptables_" tag="ptables" type="word" length="3" />
-  <variable name="cutoff_" tag="cutoff" type="cutoff_xml" dimension="1" />
-  <variable name="cross_sections_" tag="cross_sections" type="word" length="255" />
+  <variable name="verbosity_" tag="verbosity" type="integer" />
   <variable name="trace_" tag="trace" type="integer-array" />
-  <variable name="entropy_" tag="entropy" type="entropy_xml" dimension="1" />
 
 </template>