Method of non-paired spatial orbitals. Part 4.—Density matrix method for calculating wave functions with full spin projection

Abstract

A method is proposed for the calculation of NPSO wavefunctions which includes full spin projection by considering the projection of a single DODS Slater determinant in which the orbitals are obtained from the natural orbitals of the unprojected NPSO wavefunction. The method, is, in fact, a synthesis of the NPSO and alternant molecular orbital methods. The procedure has been applied to the π-electron systems of benzene, the benzyl radical, naphthalene, azulene, anthracene and phenanthrene. In general the energies are lowered beyond those obtained in previous NPSO calculations, the spin density matrix for the benzyl radical is in agreement with that obtained by the AMO method but the first order density matrices for polycyclic hydrocarbons are not improved. A refinement of the method to remove this deficiency is suggested.