diff --git a/src/physics.F90 b/src/physics.F90
index 5d3433f3be..3460873e55 100644
--- a/src/physics.F90
+++ b/src/physics.F90
@@ -1164,7 +1164,7 @@ contains
       ! calculate cosine
       mu0 = rxn % adist % data(lc + k)
       mu1 = rxn % adist % data(lc + k+1)
-      mu = mu0 + (32.0_8 * xi - k) * (mu1 - mu0)
+      mu = mu0 + (32.0_8 * xi - k + ONE) * (mu1 - mu0)
 
     elseif (type == ANGLE_TABULAR) then
       interp = int(rxn % adist % data(lc + 1))