Improved quantum theory of many-electron systems. II. The basic method

A general method of obtaining accurate and useful many-electron wave functions for atoms and molecules is developed. The method involves the proper optimization of a many-electron function which is an eigen-function of total spin and which satisfies Pauli's principle. The procedure is somewhat similar to that in which one obtains the Hartree-Fock wave function by properly optimizing a Slater determinant. There are three key features of the new method: (1) The wave function remains accurate as the nuclei configuration is changed from the equilibrium configuration to that of the dissociated molecule; (2) the many-electron wave function can be interpreted in an independent-particle scheme; (3) the independent-particle states are no longer always required to be symmetry functions for the spatial symmetry group as in the Hartree-Fock method. In addition, the energy calculated using the new method is always lower than the Hartree-Fock energy, and the method is applicable to states of any total spin and number of particles. Calculations using this method are reported for the H2 and LiH molecules.