A NEW CONFIGURATION-INTERACTION ALGORITHM FOR ELECTRON-SPIN-DEPENDENTPROPERTIES

An algorithm for large full or nearly full configuration interaction e xpansions over determinants of approximate molecular wavefunctions has been extended to include the spin-orbit and the spin-spin components of the Hamiltonian. The algorithm proceeds by evaluating directly from the molecular wavefunctions a reduced number of primitive spin compon ents of the one-and two-electron densities and the corresponding contr ibutions to the matrix elements. Recurrence relationships have been de rived and exploited to generate all remaining spin components of the d ensity functions and contributions to the matrix elements from the pri mitive densities. The algorithm has been implemented and a computer pr ogram has been developed and interfaced to quantum chemistry programs for accurate molecular computations with the inclusion of the spin-dep endent energy terms. The program has been tested by evaluating a few s pin-orbit and spin-spin matrix elements for atoms and one diatomic mol ecule, and these have been compared with experiment and with the resul ts of previous computations.