[gmx-developers] Incorrectly large pressures at lambda=1 with perturbed masses

[Jochen Hub]

Dear Gromacs developers,

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/o2j7K4GK7wojayi

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

To us, this looks like a potentially severe bug which was not detected 
because we typically perturb only few atoms and not 2/3 of the all 
atoms, as we do here.

Is this issue known?

Best wishes,
Johanna and Jochen


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Prof. Dr. Jochen Hub
Theoretical Biophysics Group
Department of Physics, Saarland University
Campus E2 6, Zi. 4.11, 66123 Saarbruecken, Germany
Phone: +49 (0)681 302-2740
https://biophys.uni-saarland.de/
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