A NEW IMPLEMENTATION OF THE 2ND-ORDER POLARIZATION PROPAGATOR APPROXIMATION (SOPPA) - THE EXCITATION-SPECTRA OF BENZENE AND NAPHTHALENE

We present a new implementation of the second-order polarization propa gator approximation (SOPPA) using a direct linear transformation appro ach, in which the SOPPA equations are solved iteratively. This approac h has two important advantages over its predecessors. First, the direc t linear transformation allows for more efficient calculations for lar ge two particle-two hole excitation manifolds. Second, the operation c ount for SOPPA is lowered by one order, to N-5. As an application of t he new implementation, we calculate the excitation energies and oscill ator strengths of the lowest singlet and tripler transitions for benze ne and naphthalene. The results compare well with experiment and CASPT 2 values, calculated with identical basis sets and molecular geometrie s. This indicates that SOPPA can provide reliable values for excitatio n energies and response properties for relatively large molecular syst ems. (C) 1996 American Institute of Physics.