Noj. Malcolm et Jjw. Mcdouall, DYNAMIC ELECTRON CORRELATION - A FRAGMENTS-IN-MOLECULES APPROACH, Journal of physical chemistry, 98(48), 1994, pp. 12579-12583
We present and test a computationally economical scheme for obtaining
dynamical correlation energy corrections for complete active space sel
f-consistent field (CASSCF) wave functions. The method relies on the d
ecomposition of the chemical system into ''fragments''. By use of a lo
calized orbital description any CASSCF wave function can be transforme
d into a classical valence bond expansion. The advantage of the classi
cal valence bond expansion is that the wave function takes the form of
a superposition of fragment covalent and ionic states. The dynamic co
rrelation energy is evaluated, using a density functional method, for
each fragment state and added to the total energy according to an ''at
oms-in-molecules'' type formula. The method is tested through applicat
ion 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 butadien
e.