[Wien] have you met such question

[Steven Homolya]

On Mon, 14 Jul 2003, Yushan Wang wrote:

> I want to calculate the enthalpy difference of a  doped system. as stated
> in some literatures, I need to calculate the total energy of the system
> before and after doping. but the calculation results did not make me
> happy, the difference of the total energy is so big, definitely
> in a disagreement with the reality. who can point out the possible error
> of my calculation.
> 

This problem arises whenever you are comparing the energies of two systems
of different compositions (stoichiometries). To make a meaningful
comparison, you'll need to subtract the sum of the "free" energies of the
constituent atoms, for both systems. By "free" energy I mean energy of a
single atom in vacuum. This is calculated by putting a single atom in a
big unit cell. See Cohesive energies in the FAQs. Also note NN 20 bohr
atom-atom separation limit (bug) and fix, as discussed recently on this
list.

Also, lstart will give you ballpark figures for the free atom energies. 
You can use these initially to get some idea of whether your results are 
sensible or not.

Steve

-- 
Steven Homolya
School of Physics and Materials Engineering
Monash University, VIC 3800
Australia
Tel: +61 3 9905 3694
Fax: +61 3 9905 3637