g_tune_pme scalevdw not working for rcoulomb values smaller than original
Consider the following command: g_tune_pme -s topol.tpr -rmin 1.0 -rmax 1.4
If topol.tpr was generated with an mdp file containing a rcoulomb of 1.2, all the test runs made by g_tune_pme with a cutoff between rmin and 1.2 will fail because rvdw is not scaled properly. The scaling is only working for values larger than the original cutoff.
A better explanation here: https://mailman-1.sys.kth.se/pipermail/gromacs.org_gmx-users/2014-March/087863.html
The van der Waals radius must equal the Coulomb radius with Verlet
This patch fixes an issue that can occur when using g_tune_pme
with a Verlet pair-list .tpr input file. If the .tpr itself has
large cutoffs (e.g. 1.2 nm) and one asks g_tune_pme to scale
down the Coulomb radius, the van der Waals radius is not
scaled down with the Coulomb radius (only upscaling worked).
One ends up with an unusable .tpr file because rVdW != rCoul.
This patch ensures that van der Waals and Coulomb radii
are always equal with Verlet pair-lists. Fixes #1460. Thanks
to Joao Rodrigues for reporting the issue!