[Pw_forum] How to add air pressure on adsorption calculation using Pwscf

[Lorenzo Paulatto]

On Wednesday, October 28, 2015 04:13:41 PM 李睿 wrote:
> Hello, everyone
> I want to calculate molecules adsorption under 1Bar air pressure using
> Pwscf. Can I add air pressure in it?

No, you cannot. Gas pressure is a macroscopic variable. It does not exist in a 
zero-temperature DFT calculation. You may do a molecular dynamics simulation 
with a bunch of molecules at a certain temperature, but it is very impractical 
and deducing the pressure from volume+temperature when the volume is so small 
and includes a chemically reactive surface is not granted.

What you can do is to include its effect via a chemical potential, you can 
find a brief explanation in chapter 3.2 of my PhD thesis 
<http://urania.sissa.it/xmlui/bitstream/handle/1963/5363/paulatto_thesis.pdf?sequence=1&isAllowed=y> 
(eq. 3.12 in particular, but read all the chapter). If I remember correctly, I 
studied the problem on  "Arthur W. Adamson, Physical Chemistry of Surfaces, 
John Wiley and Sons, 1990", so if you can get a copy of the book you'll find a 
better and longer explanation.

HTH

-- 
Dr. Lorenzo Paulatto
IdR @ IMPMC -- CNRS & Université Paris 6
+33 (0)1 44 275 084 / skype: paulatz
http://www.impmc.upmc.fr/~paulatto/
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