From owner-chemistry@ccl.net Mon Sep 28 04:46:01 2009 From: "Gonzalo Jim nez-Os s gjimenez#,#unizar.es" To: CCL Subject: CCL: AIM theory with weak interactions Message-Id: <-40365-090928044349-21143-lJYZ4Kh8RU899ZGcI1a04w ~ server.ccl.net> X-Original-From: "Gonzalo Jim nez-Os s" Date: Mon, 28 Sep 2009 04:43:45 -0400 Sent to CCL by: "Gonzalo Jim nez-Os s" [gjimenez.]|[.unizar.es] Dear CCL'ers, I have some theoretical doubts about the aplicability of the Atoms In Molecules theory in systems in which the interactions between two molecular fragments are middle- or long-range and mainly related to (non-local) electronic correlation effects (dispersion) like those ocurring in van der Waals complexes. For example, I've been performing some calculations with the well-known benchmarck benzene dimer. Whereas the p-p stacked minimum is located only with correlated ab initio methods (common DFT fails and separate the two monomers), when the wavefunction (the SCF density) calculated through DFT (i.e. B3LYP) on the optimized MP2 geometry is analyzed within the AIM context (EXT94b program), exactly the same number of critical points (CP) are found with respect to those found in the MP2 wavefunction, including seven CPs between the two fragments in the interaction region. So, my question is: if common DFT (or in the extreme case, HF) cannot account for dynamic correlation effects, how these critical points related to "weak interactions" between the two fragments can be located in the density topology? Is there any physical/theoretical inconsistency on these results? Moreover, the value of density (rho) of these zero-gradient points is very similar in fully uncorrelated methods (HF), DFT (BLYP, B3LYP) and MP2. Is there any plausible explanation for these results? Thank you very much in advance, Gonzalo Jimnez-Oss University of Zaragoza