It has been demonstrated that the soft Coulomb hole method is a reliable and efficient approach to calculate the electron correlation energy for atoms and molecules. In this method the perturbation operator −e−ωr122/r12 is introduced, where ω determines the size of the Coulomb hole. The set of parameters for ω has been redetermined to calculate equilibrium bond distances. Calculations have been performed for 41 homo- and heteronuclear singlet diatomic molecules and ions (X 1Σ+), including atoms of the second and third periods of the periodic table. The soft Coulomb hole correlation energies are compared to the corresponding empirical correlation energies. In general, calculated equilibrium bond distances are in better agreement with the experimental values than Hartree-Fock and Mo/ller-Plesset-2 results. With respect to Mo/ller-Plesset-3, the soft Coulomb hole method gives slightly larger values for the average deviations, except for the homonuclear series.

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ABSTRACT:The soft Coulomb hole method introduces a perturbation operator, defined by Ϫe Ϫr 12 2 /r 12 to take into account electron correlation effects, where represents the width of the Coulomb hole. A new parametrization for the soft Coulomb hole operator is presented with the purpose of obtaining better molecular geometries than those resulting from Hartree-Fock calculations, as well as correlation energies. The 12 parameters included in were determined for a reference set of 12 molecules and applied to a large set of molecules (38 homo-and heteronuclear diatomic molecules, and 37 small and medium-size molecules). For these systems, the optimized geometries were compared with experimental values; correlation energies were compared with results of the MP2, B3LYP, and Gaussian 3 approach. On average, molecular geometries are better than the Hartree-Fock values, and correlation energies yield results halfway between MP2 and B3LYP.