Hello all,
I am having an issue with a coexistence study involving a gas phase and a solution phase. My box is a rectangular prism with the gas phase as a “slab”. The key issue is that the pressure tensor elements are not equal on the diagonal. With only fluid phases, one would expect that Pset =
= = = , where Pset is what pressure we set our barostat to and the bracketed Ps correspond to the average pressures (overall and in the x, y, z dimensions, respectively). However, the results that I am seeing indicate that
is sometimes close to Pset, but that is often greater than Pset (though not always) and that = < Pset. The x dimension is longer than the other two dimensions (by ~ a factor of 3), as well as being the direction which is normal to the gas/solution interface. I have tried several different barostatting scenarios; in general all give the aforementioned pressure relationships. These include:
fix 1 all npt temp $T T {Tdamp} x $p p {Pdamp}
(only the x box dimension is changed)
fix 1 all npt temp $T T {Tdamp} iso $p p {Pdamp}
(x, y, and z box dimensions are changed, each by same percentage)
Finally, the only simulation that I have gotten to successfully maintain the pressure elements as approximately equal is one in which I used anisotropic pressure control:
fix 1 all npt temp $T T {Tdamp} aniso $p p {Pdamp} couple yz
(x, y, and z dimensions are all changed, and can change independently)
However, the anisotropic case has the unfavorable characteristic that my box aspect ratios get severely distorted (x dimension becomes very long at the expense of y and z, probably to reduce the interfacial area between the two phases).
Probably the strangest part is that this inequality of pressure elements occurs even for NVT conditions
fix 1 all nvt temp $T T {Tdamp}
I am trying out some other scenarios right now, but these results seem odd. Do you have any idea why they might be occurring and how they might be fixed?
Thank you very much, I greatly appreciate any insight,
Brandon
p.s. I’ve attached a sample “in” file with forcefield file and restart file
in.coex (891 Bytes)
MeWa.sw (800 Bytes)
prod2.restart (688 KB)