Subject: Solid-Gas Thermodynamics from DFT

From: Aaron Deskins <ndeskins/at/purdue.edu>
Date: Fri, 27 Feb 2004 09:52:12 -0500

Hello everyone,
  I had some questions about extracting thermodynamics information from 
DFT results. Specifically I'm using the VASP periodic code to model the 
adsorption of gases onto a solid surface (A + Solid --> A-Solid).
 
  A paper by Sohlberg et al. (Sohlberg, Pantelides, and Pennycook. J.Am. 
Chem. Soc. 2001, 123,6609-6611) examined the incorporation of hydrogen 
into bulk ceria (H-Ceria --> H2 + Ceria). They began with the free energy:
delG = delH - TdelS.
 For the entropy term they neglected the solid-phase entropy since 
Ssolid << Sgas, and they give a reference for this assumption. They then 
assume that delH ~ delE. So delG becomes delG = delE -T*Shydrogen. They 
take their delE value from DFT results and Shydrogen from a tabulated 
values. This provides a nice way to find how the reaction proceeds as a 
function of temperature.

  So my question is this - is the assumption delH~delE a valid 
assumption? If so, are there any references to this assumption that 
would help me?
 
  A similar approach to find solid-gas reactions as a function of 
temperature (and pressure) is taken by Arrouvel and his colleagues in 
another paper (Journal of Catalysis, 222, (2004), 152-166), though with 
slightly different equations.

  Any help is appreciated.
   
Aaron Deskins
Purdue University