Re: Pressure Discrepancy

From: Morad Alawneh (
Date: Wed Mar 28 2007 - 15:00:51 CDT

*Dear Richard,

My system basically having protein surrounded by a lipid bilayer with
water and salt. So to have the right properties for those lipids, I have
to have a box area that gives the right lipids properties, which is
after equilibrating that system under NPgT.

NPAT is a special case of NPT, which maintain a fixed cell area, e.g. x
and y box dimension are kept fixed whereas its length is kept varying.

Since there is no way in physics to have NPVT, and according to the
phase transition and from the ideal gas law PV = NRT, there is no way to
have 3 independent variables in any system, you can have two independent
variables and the third one is a function of the other two. If we keep
every thing constant, the system will explode, and we will get an energy

Yes, under NPAT or NPT, the cell does not have a constant volume, and
since both the pressure and the temperature are not exactly fixed, the
cell dimensions keep varying.

Even if you have NVT, both the temperature and the pressure will vary,
whereas N and V are not.


Morad Alawneh*

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