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Created by Paul Bauer@acmnpvOwner

PME + LJ-PME with unperturbed charges / vdW lead to zero forces from the reciprocal space - Redmine #3359

Archive from user: Shun Sakuraba

In issue #2640, there was a problem when PME + LJ-PME are enabled and only charges or vdWs are perturbed, and was fixed in revision 12f01f79 . However the “fix” actually seems to nullify all the forces from the reciprocal space to the perturbed atoms.

In an attached simple system with only two atoms, the force between two atoms disappear only when PME + LJ-PME is enabled. Steps to reproduce is:

gmx_d grompp -f minimal_fep1.mdp -c minimal.gro -o minimal_fep1_out
gmx_d mdrun -deffnm minimal_fep1_out -rerun minimal.gro
gmx_d dump -f minimal_fep1_out.trr

The force along x should be like 33kJ/mol/nm, but is 4e-5 instead (as of ver. 2019.5)

The reason seems to be the confusing scaling factor of the lambda value (pme.cpp, line 1440).
The correct fix seems to be trivial (patch attached). Like the original issue, the case only rarely happens and only reciprocal part is nullified, so the affected users should be minimal.

(from redmine: issue id 3359, created on 2020-01-29 by gmxdefault, closed on 2020-02-05)

  • Changesets:
    • Revision a507acbc by Berk Hess on 2020-02-04T10:05:04Z:
Actually fix PME forces with FE without perturbed q/LJ

PME would incorrectly ignore the mesh forces on perturbed atoms when
no charges or LJ atom types were actually perturbed.
This is because the "fix" in commit 12f01f79 incorrectly changed the
lambda value used for the PME forces from the actual lambda value
to 1 instead of to 0 as it should have been.

Fixes #2640
Fixes #3359

Change-Id: Ib19565bf017ddbd2fddc66c52be0ff9efc7e707c
  • Revision 7387a098 by Berk Hess on 2020-02-05T07:14:49Z:
Actually fix PME forces with FE without perturbed q/LJ

PME would incorrectly ignore the mesh forces on perturbed atoms when
no charges or LJ atom types were actually perturbed.
This is because the "fix" in commit 12f01f79 incorrectly changed the
lambda value used for the PME forces from the actual lambda value
to 1 instead of to 0 as it should have been.

Fixes #2640
Fixes #3359

Change-Id: Ib19565bf017ddbd2fddc66c52be0ff9efc7e707c