[gmx-developers] wrong dhdl with lj-pme

[Igor Leontyev]

Dear gmx developers,

In solvation free energy simulations with gmx 2020.1 I came across the following issue. vdW component of dG and DHDL values are ~2X overestimated at the lambda region in which vdw-lambdas are changing. Additional testing has shown that this issue is related to "vdw-type = PME."


I have compared (for lambda-state=4) <dVvdw/dl> results of gmx2020 with vdw-type: PME and CutOff, with the result of old gmx5.1 with vdw-type = PME. All was run on CPU for 5 nanosecs with PME electrostatics. It turned out that gmx2020 with vdw-type=CutOff reproduces (within the accuracy) the <dVvdw/dl> result of gmx5.1 with vdw-type = PME, which is expected. In the uniform environment both vdw-types should give accurate solvation FE. The result of gmx2020 with vdw-type=PME, however, is around 2X exaggerated, which seems to be a bug.


Here are the numbers:

              gmx-ver/vdw-type       <dVcoul/dl>      <dVvdw/dl>

              2020/PME                    -6.39450e-01    9.72647e+01

              2020/CutOff                 -3.99985e-01    4.39876e+01

               5.1/PME                    -3.08675e-01    4.36738e+01


Note, the problem is related to DHDL only, all potential energy components (including vdW) in all runs are consistent. Let me know if you need input files to reproduce the issue. Though I believe, it can be observed for any simple solute in water solution.

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