From korkin@qtp.ufl.edu Fri Apr 4 13:33:59 1997 Received: from qtp.ufl.edu for korkin@qtp.ufl.edu by www.ccl.net (8.8.3/950822.1) id NAA11961; Fri, 4 Apr 1997 13:33:11 -0500 (EST) Received: from orange by qtp.ufl.edu (SMI-8.6/SMI-SVR4) id NAA02130; Fri, 4 Apr 1997 13:32:54 -0500 Date: Fri, 4 Apr 1997 13:32:54 -0500 (EST) From: Anatoli Korkin Reply-To: Anatoli Korkin Subject: Re: CCL:electronic correlation To: Sylvie.Joanteguy@univ-pau.fr Cc: chemistry@www.ccl.net, rjbgroup@qtp.ufl.edu Message-ID: MIME-Version: 1.0 Content-Type: TEXT/plain; charset=us-ascii Content-MD5: sh/dj0ugqIiFPQIqh9WbKA== X-Mailer: dtmail 1.2.0 CDE Version 1.2_22 SunOS 5.6 sun4d sparc > > Dear CCL, > > My question is short: What's exactely the electronic correlation (dynamic and no dynamic correlation)? > > Many thanks for your help > > Sylvie Joanteguy > Dear Silvie, As you as a short question, I will give you a short answer :-) The definition of the electronic correlation is simple - it is what you get above single-determinant SCF method. Of course, you should remeber that your result depends of the basis set you use. Only full CI with infinite number of basis functions will give you an exact solution for the energy and the wave fuction within Born-Oppenheimer approximation. The definition of the dymanic and non-dynamic correlation is less accurate. If you have a good SCF solution, and you do a perturbation calculation on top of it, your correction to the energy, structure and other properties is considered to be an effect of dynamic correlation. If you need a few determinants to get a good first approximation for the wave function, it is what people call non-dynamic correlation. Of course, the destingtion between "good" and "bad" is rather uncertain, as it happens to all qualitative concepts. The coupled cluster (CC) method [1] provides both dynamic and non-dynamic correlation effects, and if the number of electrons is equal to the level of CC (e.g. CCSDT for 3 electrons) you will get the total correlation, as the full CI does. Anatoli Korkin [1] see, for example, R.J. Bartlett & J.F. Stanton, Applications of Post-Hartree-Fock Methods: A Tutorial, in Reviews in Computational Chemistry, Vol. 5, VCH Publishers, 1994)