DYNAMIC ELECTRON CORRELATION - A FRAGMENTS-IN-MOLECULES APPROACH

We present and test a computationally economical scheme for obtaining dynamical correlation energy corrections for complete active space self-consistent field (CASSCF) wave functions. The method relies on the decomposition of the chemical system into ''fragments''. By use of a localized orbital description any CASSCF wave function can be transformed into a classical valence bond expansion. The advantage of the classical valence bond expansion is that the wave function takes the form of a superposition of fragment covalent and ionic states. The dynamic correlation energy is evaluated, using a density functional method, for each fragment state and added to the total energy according to an ''atoms-in-molecules'' type formula. The method is tested through application to bond dissociation in H-2 and LiH and also to the evaluation of barrier heights for the reactions Cl + HCl --> HCl + Cl and CH3 + CH4 --> CH4 + CH3 and the Diels-Alder reaction between ethene and butadiene.