NONLINEAR ROOTHAAN EQUATIONS

In the regime of the molecular-orbital theory, a new approach beyond o f the Hartree-Fock approximations is proposed. By an appropriate intro duction of the quantum fluctuation into the molecular electronic groun d state, the Hartree-Fock equations plus Gaussian fluctuation are obta ined. Basically, the procedure proposed consists of an adequate utiliz ation of the Gaussian approximation of the Ising model on a lattice (K . G. Wilson et al., Phys. Rep. C 12, 76 (1974)). It leads to an effect ive nonlinear Hamiltonian for the molecule for studying nonlinear prob lems in the molecular electronic structure. (C) 1993 John Wiley & Sons , Inc.