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)