A b i n i t i o SCF and CI studies on the ground state of the water molecule. I. Comparison of CGTO and STO basis sets near the Hartree–Fock limit

A b initio self-consistent field and configuration interaction calculations have been carried out for the ground state of the water molecule near its experimental equilibrium geometry. Several contracted Gaussian (CGTO) and Slater type (STO) basis sets have been used, and the effects of variations in the type and size of the basis set on the SCF and CI energies and on computed first-order one-electron properties have been examined. The CI calculations included all single and double excitation configurations relative to the SCF function. The largest basis set studied, a 39-STO (5s4p2d/3s1p) basis, provided an SCF energy of −76.064226 hartree, which is about 3 mhartree above the estimated Hartree–Fock limit, and a CI energy of −76.339802 hartree, which accounts for 74.5% of the estimated total correlation energy. The SCF and CI one-electron property expectation values for this basis are generally within 5%–10% of the experimental values. Ionization potentials, binding energy, and pair-correlation energies have also been computed with the 39-STO basis.