Study of the electronic structure of molecules. XXIII. Decomposition of correlation energy into atomic, covalent, ionic and van der Waals components

A new and general algorithm is presented to obtain the correlation energy i n molecular systems using a non-conventional density functional to scale th e Hartree-Fock integrals. The correlation energy is partitioned into the su m of the atomic correlation energies at dissociation, and correlation contr ibutions from covalent and ionic bonds and from van der Waals interactions. The method is validated by computing 101 closed-shell molecules, 8 radical s, 5 molecular ions, 2 H-bonded and 6 van der Waals systems. The technique requires only a few additional self-consistent-held iterations after HF con vergence, thus it is computationally simple and fast. By using a functional with three empirical parameters, we recover 98.45% of the total correlatio n energy with 8.02 kcal/mol average deviation in the computed atomisation e nergy; with more parameters, these values become 99.58% and 2.25 kcal/mol a verage deviation, respectively. (C) 2001 Elsevier Science B.V. All rights r eserved.