Why the density of water is fluctuating like this in the electric field？

User discussions
1

GROMACS version:
GROMACS modification: Yes/No
Here post your question density of water

the black is the density of water at 534 K, 14 bar ( equilibrated by Parrinello-Rahman barostat )
the red is the density of water under the effect of a electric field (1*10^9 V/m 2.5GHz) in the same temperature and pressure

2

by the way, I also found the average value of density was increase from 808 kg/m3 to 834 kg/m3
the potential energy of water shows the same trend.
potential energy

sorry for uploading the wrong pictures

3

Hi Silentium,

Indeed it’s a bit strange, if I calculate correctly, with your frequency change you would expect to see fluctuations in the range of 2.5 ns.

I would expect some fluctuations in potential energy due to the extra work exerted on the system from the electric field.

There is two issues, that I can imagine contribute to what you see:

  • the pressure coupling algorithm might not be able to keep up with the fluctuations in the system due to possibly high forces; you could try changing the coupling time. In GROMACS2021 (due to be released soon) you will be able to use stochastic cell rescaling that might be able to handle this better, however for currently released versions you will be bound to checking coupling time
  • in some GROMACS2020 before 2020.5 versions there were issues with the electric field:

The electric field generated by the electric field module would be incorrect when used together with domain decomposition due to an error with indexing the field to all atoms instead of just those on the current domain.

In overlap regions between domains, which have the thickness of the pairlist cut-off distance, the electric field would be doubled (or more with 2D or 3D domain decomposition).

To validate if a simulation has been affected by the issue, users should calculate the actual potential across the simulation box using the Poisson equation. If this potential agrees with the one provided as the input, a simulation was not affected.

Issue 3800

You might have suffered from this issue (more details here: Bug with electric field simulation using gromacs2020 (#3800) · Issues · GROMACS / GROMACS · GitLab)