COMPARISON OF PERTURBATIVE AND MULTICONFIGURATIONAL ELECTRON PROPAGATOR METHODS

Ionization energies below 20 eV of 10 molecules calculated with electr on propagator techniques employing Hartree-Fock orbitals and multiconf igurational self-consistent field orbitals are compared. Diagonal and nondiagonal self-energy approximations are used in the perturbative fo rmalism. Three diagonal methods based on second- and third-order self- energy terms, all known as the outer valence Green's function, are dis cussed. A procedure for selecting the most reliable of these three ver sions for a given calculation is tested. Results with a polarized, tri ple zeta basis produce root mean square errors with respect to experim ent of approximately 0.3 eV. Use of the selection procedure has a slig ht influence on the quality of the results. A related, nondiagonal met hod, known as ADC(3), performs infinite-order summations on several ty pes of self-energy contributions, is complete through third-order, and produces similar accuracy. These results are compared to ionization e nergies calculated with the multiconfigurational spin-tenser electron propagator method. Complete active space wave functions or close appro ximations constitute the reference states. Simple field operators and transfer operators pertaining to the active space define the operator manifold. With the same basis sets, these methods produce ionization e nergies with accuracy that is comparable to that of the perturbative t echniques. (C) 1996 John Wiley & Sons, Inc.