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

Jochen Hub jochen.hub at uni-saarland.de
Tue Mar 2 09:35:30 CET 2021

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:


(The relaxation in the problematic cases is due to rapid box expansion).

We have a test system here:


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

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

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