[gmx-users] constant area and normal pressure

[Erik Lindahl]

Hi,

> 
> pcoupl                 = Parrinello-Rahman
> pcoupltype              = anisotropic
> tau_p                   = 0.5
> compressibility         = 0.00 0.00  4.5e-5 0.0 0.0 0.0
> ref_p                   = 0.00 0.00 0.00 0.00 0.00 0.00
> 
> or something similar with semiisotropic coupling.  Would this constitute a
> proper constant area and constant normal pressure (i.e. NP[n]AT ) ensemble?
> 

Yep, it should. To nitpick you should probably couple the z-direction to 
1.0 bar, but with the fluctuations on a molecular level this is 
essentially the same as 0.0 :-)

I would probably select a larger time constant than 0.5 too, something 
in the order of 2-10 ps. With Parrinello-Rahman, this will be the period
of the oscillations at equilibrium pressure, and 0.5 is short enough 
that it could affect the individual atom motions significantly.

Let me know how it works; I've mainly tested P-R on crystal systems with 
a completely flexible box. It should work for your case too, but 
membranes can be quite sensitive to pressure coupling.

Cheers,

Erik