## Summary Changing a single atom type at the lambda = 1 endpoint affects the short-range electrostatics at lambda = 0 using GROMACS 2022.4 (it shouldn't). ## Exact steps to reproduce Using the two folders in [these input files](/uploads/e8704e0e5587a4bec0ed910b0bf1d47d/Archive.zip), one obtains different potential energies for the short-range electrostatics (using `gmx energy -f gromacs.edr`): input0: 13.0602 kJ/mol input1: 13.0313 kJ/mol The *only* difference between the two inputs is a single line in the topology file. For input0 this is: `7 C2 1 LIG C7x 7 -0.054060 12.010780 C2 -0.054060 12.010780` And for input1 this is: `7 C2 1 LIG C7x 7 -0.054060 12.010780 C2_du -0.054060 12.010780` ## If this is a bug, (1) what happens, and (2) what did you expect to happen? Since both MDP files have this section: ``` fep-lambdas = 0.00000 0.00000 coulomb-lambdas = 0.00000 1.00000 init-lambda-state = 0 ``` The energies should not be affected because of this change, because 1. changing the atom type should not affect the electrostatics and 2. changing something at lambda = 1 should not affect the energies at lambda = 0. ## Relevant input files, logs and/or screenshots All attached. This was run on macOS. ## Possible fixes This seems to be related to an issue that was fixed in 2022.4. I would be interested in learning how to circumvent it if possible.