Limited Expansion Gaussian Lobe Orbitals in Ab Initio Studies of Atoms and Molecules

Results of ab initio calculations for first-row atoms and ions and LCAO–MO calculations for methane, fluoromethane, difluoromethane, trifluoromethane, and ethylene are presented for basis sets consisting of groups of limited numbers of s-type Gaussians and simple Gaussian lobe p orbital representations. Parameters are presented for atoms and ions of elements beryllium through fluorine using a three-Gaussian 1s-core group, an additional Gaussian for the 2s orbital and a two-Gaussian lobe p orbital. When used in LCAO–MO calculations this atomic orbital representation appears to provide results for charge distributions, molecular geometries, valence shell MO energies, calculated HF dissociation energies, and excitation energies (for ethylene) comparable to results obtained using minimum STO basis sets. A method for isolating the core orbitals from the LCAO variation process is applied and discussed for the molecular calculations.