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["Zhang, Waigen" <wg.zhang@okstate.edu>]

Hi,Paul,
Firstly,thank you for your reply.
Maybe the difference is from the pseudopential.But what I need is to find a 
relationship between total energy and volume(pressure).Therefore,the exact 
energy
is important for me.
One thing I want to know most is that which result should I choose and should 
I use
GGA in other calculation for consitent,or LDA is OK?

Best,
Zhang,Waigen


-----Original Message-----
From: paul.boulanger@umontreal.ca [mailto:paul.boulanger@umontreal.ca]
Sent: Tue 4/17/2007 2:50 AM
To: forum@abinit.org
Subject: Re: RE: [abinit-forum] LDA and GGA
 
Hi,

I don't really understand this big difference in total energy, but maybe the 
key to comprehension comes from the two different pseudopotentials. In fact, 
two different pseudopotentials should give two different total energies, we 
can change the number of electrons, change the interaction between core and 
valence, etc.

After all, a pseudopotential is not suppose to conserve total energy but to 
conserve the physics. All physical quantities are variation of the total 
energy: binding energy, bulk modulus, phonon band structure, response 
functions, etc. Hence comparing only total energies is a bad test to judge 
the quality of a pseudopotential or for functionals. One should compare 
physically mesurable quantities.

Could you provide the two pseudopotential files (and source files)? Could you 
calculate the binding energy or different propertie of your system with LDA 
and GGA? These different quantities should give better agreement (but some 
differences are expected, if I remember correctly GGA tend to overestimate 
the binding). 

Good luck,
Paul

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