Incorrectly large pressures at lambda=1 with perturbed masses
We get incorrectly large pressures in FEP simulations in the lambda=1 state if we perturb the masses of all hydrogen atoms. Specifically, we simulate a pure-water box and perturb the masses of all H atoms from 1amu to 2amu. This happens with Gromacs 2019.6, 2020.5, and 2021, but NOT with 2018.6.
The pressures are shown in a PDF here:
https://cloud.hiz-saarland.de/s/NH6Mz8QKi39qBH7
(The relaxation in the problematic cases is due to rapid box expansion).
We have a test system here:
https://cloud.hiz-saarland.de/s/oJXPp5BRnTgRxg9
This does NOT happen if:
- masses are not perturbed
- using Gromacs 2018.6
- the same state is defined to lambda=0 by modifying the topology (use heavy.top with test_lambda0.mdp in the tar.gz)