On Sat, 2009-06-06 at 00:20 -0700, ali kazempour wrote: > Dear all > I have a question about charge simulation.
> In common DFT simulation with neutral charge we have Ewald sum that > describe charge-charge interaction . This sum is not problematic . But the ewald sum is a _method_ to efficiently compute (or rather approximate) the _coulomb contribution_ to the total energy per unitcell in a crystal. > when we simulate for example ionization energy, we have to use > correction like makov-payne . In both case we have charge -charge > interaction . What is the differences between two case? if you take out an electron, you will have a charged unit cell, and due to periodic boundary conditions, this will happen in _every_ unitcell. this is effectively an unphysical situation and such a crystal is not stable. however, you can still do calculations of such systems, if you ignore the g=0 term in the ewald sum. this will lead to an error in the total energy, but it is independent of the positions of the atoms or ions and thus the forces will still be correct. cheers, axel. > Thanks a lot > > Ali Kazempour > Physics department, Isfahan University of Technology > 84156 Isfahan, Iran. Tel-1: +98 311 391 3733 > Fax: +98 311 391 2376 Tel-2: +98 311 391 2375 > > > > _______________________________________________ > Pw_forum mailing list > Pw_forum at pwscf.org > http://www.democritos.it/mailman/listinfo/pw_forum -- ======================================================================= 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 ======================================================================= If you make something idiot-proof, the universe creates a better idiot.
