Elementary Jacobi rotation method for generalized valence bond perfect-pairing calculations combined with simple procedure for generating reliable initial orbitals

A simple procedure is presented for generating reliable starting orbitals for the strongly orthogonal perfect-pairing (PP) generalized valence bond (GVB) calculations. The initial ist GVB natural orbitals (NOs) are taken from the occupied localized molecular orbitals (LMOs), which are obtained by the Pipek-Mezey population localization procedure. The initial 2nd GVB-NOs, which are localized in the same regions as the corresponding Ist GVB-NOs, are taken from the virtual orbitals optimized so as to maximize the exchange integrals pertaining to the occupied LMOs. The elementary Jacobi rotation approach is then presented to calculate the GVB-SCF orbitals. In conjuction with the present procedure of initial guess, it gave fast and reliable convergence in the GVB-PP calculations for the various basis sets and the arbitrary numbers of GVB pairs not only in equilibrium but also for those with distorted geometries. The convergence rate of this approach was independent of the number of correlated GVB pairs, the presence of lone pair GVB orbitals and geometry distortion. (C) 2000 Elsevier Science B.V. All rights reserved.