From owner-chemistry@ccl.net Thu Feb 9 07:14:01 2006 From: "Pierre Archirel pierre.archirel],[lcp.u-psud.fr" To: CCL Subject: CCL: ab initio and DFT Message-Id: <-30827-060209065705-6504-QB42uEgMSeBpRNGmPa3poQ:_:server.ccl.net> X-Original-From: Pierre Archirel Content-Transfer-Encoding: 8bit Content-Type: text/plain; charset=ISO-8859-1; format=flowed Date: Thu, 09 Feb 2006 12:03:56 +0100 MIME-Version: 1.0 Sent to CCL by: Pierre Archirel [pierre.archirel~!~lcp.u-psud.fr] Dear collegues, Ab initio and DFT methods can be nicely differentiated with the following statement: 1- in ab initio we correlate the wave-function 2- in DFT we correlate the hamiltonian This can be seen as follows: E_exact= (no doubt about it...) Now if Psi_D is a simple determinant, it can be transformed to the exact wave function: Psi_exact=U Psi_D and you get: E_exact== with H_mod=aU H_exact U This means that the exact energy can be obtained with a simple determinant and a modified hamiltonian. DFT is looking for this new hamiltonian. Note that this is also the spirit of semi-empirical theories, who also postulate that the wave-function is a simple determinant, and parametrise H, so as to get the exact energy. DFT and semi-empirical methods both correlate the hamiltonian. Best wishes, P.A. __________________________________________________________ Pierre Archirel Groupe de Chimie Théorique Laboratoire de Chimie Physique Tel: 01 69 15 63 86 Bat 349 Fax: 01 69 15 61 88 91405 Orsay Cedex France pierre.archirel-x-lcp.u-psud.fr __________________________________________________________