Fix calc_grid avoiding PME grid factor 84

Due to an indexing issue, it was unlikely that calc_grid, and thus
grompp, would choose a PME grid of 84*2^n and instead use a factor 96.
For a cubic box, this could increase the FFT cost by a factor 1.5.
Fixes #1605.

Change-Id: I8d6a85a98267af053b66867722fc5626ef907964

diff --git a/src/gromacs/gmxlib/calcgrid.c b/src/gromacs/gmxlib/calcgrid.c
index 3d1e70d909ef0bb0932bbd80df9cd12976c46120..81856d489c877e5d0e97fad3423a90fecc815b77 100644 (file)
--- a/src/gromacs/gmxlib/calcgrid.c
+++ b/src/gromacs/gmxlib/calcgrid.c
@@ -137,7 +137,7 @@ real calc_grid(FILE *fp, matrix box, real gr_sp,
                 /* Determine how many pre-factors of 2 we need */
                 fac2 = 1;
                 i    = g_baseNR - 1;
-                while (fac2*grid_base[i-1] < nmin)
+                while (fac2*grid_base[i] < nmin)
                 {
                     fac2 *= 2;
                 }