[gmx-users] Re: Liquid/Gas Systems

[Vitaly Chaban]

>
> You had helped me earlier on calculating the heat of vaporization of
> methanol and it worked great.  I'm just trying hard to understand
> conceptually what is the difference between simulating a liquid phase
> and a gas phase in Gromacs.  I mean technically if we throw in 1000
> molecules of component A, would it only be a gas if we made the box
> super huge?  If we run NPT on a system, is that technically when we
> are compounding it together to find a liquid density?  I'm just a bit
> confused on the difference.  I've been simulating liquids up to this
> point so when I ran only NVT on a single gas molecule, I was trying to
> understand why we only run NVT on it (0 pressure so no pressure
> coupling).


I would say the things are much easier than you try to understand
them. There is no difference between simulating different phases in MD
at all. Generally, you may try NPT to simulate gaseous phase. BUT: you
should start from already equilibrated  gas phase, since barostats do
not perform adequately if the pressure/volume drift a lot during MD.
This is not a problem of MD itself.


Dr. Vitaly V. Chaban, Department of Chemistry
University of Rochester, Rochester, New York 14627-0216