From bouyer@ext.jussieu.fr Wed Apr 26 11:47:25 1995 Date: Mon, 24 Apr 1995 18:47:13 +0100 From: Frederic BOUYER To: desmond@trigger.ch.umist.ac.uk Subject: Re: CCL:Charges/bond-breaking Dear Simon, If you are using HF(Hartree-Fock) or DFT methodology, I will recommend UHF calculations, since like H2 (see A. Szabo et N. S. Ostlund, Modern Quantum Chemistry, Introduction to Advanced Electronic Structure Theory, Mc Graw-Hill, New-York, 1989), one single electron is localized on each atom. A RHF calculation will produce an overestimated energy (beginning a given interatomic distance, not at the stable structure) since it will force the two electrons (for H2) to be in the same molecular orbital. I will recommend too to take into account the effect of the correlation: MP2 (or n) or CI (if you can) for Hartree-Fock; for DFT techniques, correlations are taken into account (even at VWN level of calculations), the best will be (I think) Becke+Lee-Yang-Parr (gradient correction methods). The aim is to take into account different possible states (the fact that one electron could be on one level of energy or one other, with the HF point of view) in the global wave function. Take the same procedure to calculate all your potential energy surfaces, so that all energies can be comparable, for calculating energy reactions, or energy barriers for the dissociations. That is all I can say, at the moment, roughly speaking. Tell me if I am wrong. I am very interested in sharing with you, if it is possible, all the replies you could receive. Thank you in advance. Regards, Frederic Bouyer ______________________________________________________________________________ Frederic BOUYER Laboratoire d'Electrochimie et de Chimie Analytique Equipe "Reactivite en Milieux Ioniques Liquides" (G. PICARD) ENSCP 11, rue Pierre et Marie Curie 75231 PARIS Cedex 05 FRANCE Tel : (33)-1-43-54-53-84 ou (33)-1-44-27-67-51 ou (33)-1-44-27-66-94 Fax : (33)-1-44-27-67-50 E-mail : bouyer@ext.jussieu.fr http://alcyone.enscp.jussieu.fr/ created, in progress, under construction ... http://alcyone.enscp.jussieu.fr/Pages/LECA/GP/frederic.html ---------------------------------------------------------------------------