[CPMD-list] chemical potential and fermi level

[Axel Kohlmeyer]

On Tue, 19 Sep 2006, fafa chiker wrote:

FC> Hi

dear fafa,

FC>   I am a new user on cpmd code, and I try to study the formation
FC> energies of point defects in GaAs on different Supercell (8atoms,
FC> 64-atoms. 216-atoms), so I find some problems and difficulties to do
FC> this, please there are some one who can answer me to well understand
FC> the computing program, my questions are:

since you are new to cpmd, i would recommend you also post an input
file example, so we can more easily see whether a problem you are
seeing is related to input file errors (they do happen to everybody
at the beginning, a job that finishes does not warrant a correct
answer etc.). it would also help if you mention the version of
CPMD that you are using and sometimes also the platform that you
are running on (e.g., there are a few compiler versions that 
are known to miscompile numerically intensive code).

FC> 1. How can I determine the Fermi level of any Supercell or simple cell?

if you understand the fermi level as the energy of the highest
occupied band, you'll find it in the output as chemical potential.
you have to enable diagonalization to get that output.

FC> 2. In calculating the chemical potentials for different bulk
FC> materials (GaAs, Ga, As), and using different cutoff, I find
FC> different values of Chemical potential and different eigenvalues for
FC> occupied orbitals.

eigenvalues (and thus the chemical potential, see above) converge
rather slowly with the basis set size (= plane wave energy cutoff). 
however, for as long as you are using the same conditions (FFT grid, 
basis set, box size, etc.) you should get a significant amount of
error cancellation. please check the mailing list archives and you'll
see that there are several people mentioning the conditions where
one can compare energies and related properties.

cheers,
   axel.

FC> best regards
FC> fafa
FC> 
FC> 
FC> 
FC>  		
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-- 
=======================================================================
Axel Kohlmeyer   akohlmey at cmm.chem.upenn.edu   http://www.cmm.upenn.edu
   Center for Molecular Modeling   --   University of Pennsylvania
Department of Chemistry, 231 S.34th Street, Philadelphia, PA 19104-6323
tel: 1-215-898-1582,  fax: 1-215-573-6233,  office-tel: 1-215-898-5425
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If you make something idiot-proof, the universe creates a better idiot.