about "external pressure"

Message

hehui · #1 Post by **hehui** » Mon Aug 07, 2006 1:22 am

Hi
I want to know what is the meaning of "external pressure" and the influence on the free energy.
Thanks

#2 Post by **admin** » Mon Aug 07, 2006 9:02 am

the external pressure corresponds to the hydrostatic pressure which yields a cell of the given volume (as calculated from the trace of the stress tensor)

d-farrell2 · #3 Post by **d-farrell2** » Thu Nov 13, 2008 10:26 pm

what is the sign convention used for the external pressure? (haven't been able to find this in the documentation anywhere)

in other words: does a negative external pressure mean the atoms in the system are subjected to a compressive load?

d-farrell2 · #4 Post by **d-farrell2** » Sat Nov 15, 2008 2:00 pm

OK, I was able to do some quick calculations to find this:

(be sure you have PREC = HIGH or at the very least MED or else the forces and and therefore stress tensor won't be converged... despite what the docs say about LOW being sufficient for MD)

'external pressure' < 0 = tension of cell (i.e. cell larger than equilibrium value)
'external pressure' > 0 = compression of cell (i.e. cell smaller than equilibrium value)

fanghz · #5 Post by **fanghz** » Mon Dec 29, 2008 1:41 pm

Dear Admin,

Sorry I don't understand what you meant by this sentence:

"the external pressure corresponds to the hydrostatic pressure which yields a cell of the given volume (as calculated from the trace of the stress tensor) "

Did you mean the external pressure results from the discrepancy in the calculated cell volume and the equilibrated one ?

If I want to apply a certain pressure on a cell, should I set the "PSTRESS" vaule or adjust the volume of the cell artificially ? Besides the latter may result in an unpredictable pressure, both methods have any differences in nature ?

I really want to make these things clear. Thanks for the answer!

d-farrell2 · #6 Post by **d-farrell2** » Tue Dec 30, 2008 6:11 pm

The external pressure is a result of the simulation cell not being the same size as the equilibrium cell, so the contents of the cell are subject to some non-zero pressure. It isn't a discrepancy, it is physical.

I adjust the volume of the cell by scaling it isotropically myself. PSTRESS only seems to apply when you are trying to calculate the O K equilibrium size of the cell under a known pressure.