COMPARISON OF THE PERTURBATIVE CONVERGENCE WITH MULTIREFERENCE MOLLER-PLESSET, EPSTEIN-NESBET, FORCED DEGENERATE AND OPTIMIZED ZEROTH ORDERPARTITIONINGS - THE EXCITED BEH2 SURFACE

High order perturbation energies are computed for excited (1)A(1) stat es of BeH2 at geometries near the Be-->H-2 symmetric insertion transit ion state. The equations of multireference perturbation theory are sol ved through 30th order to study the difficulties in selecting the appr opriate zeroth order Hamiltonian, orbitals, orbital energies, and refe rence functions for the computations of smooth molecular potential ene rgy surfaces. The origin of the perturbative divergence produced by Mo ller-Plesset and Epstein-Nesbet partitionings is analyzed using a conc eptually simple two-state model constructed using one state each from the reference and orthogonal spaces. The optimized zeroth order partit ioning scheme (OPT) for double reference space computations with confi gurations 1a(1)(2)2a(1)(2)3a(1)(2) and 1a(1)(2)2a(1)(2)1b(2)(2) produc es a truly convergent perturbation expansion through 30th order. The O PT energies are accurate in low orders as compared to the exact (197 d imensional) solution within the basis. The forced valence orbital dege neracy partitioning method (FD) also generates a truly convergent expa nsion for the same double reference space calculation, with slightly p oorer low order energies than the OPT scheme. The BeH2 system facilita tes the consideration of larger reference spaces (constructed using th ree through six orbitals) where the FD method produces highly accurate energies in low orders despite the asymptotic nature of the FD pertur bation expansion. The ''delayed'' perturbative divergence behavior wit h the FD partitioning scheme (for large reference spaces) is shown to occur due to the incorrect ordering between the zeroth order energies of some reference and complementary space levels. (C) 1997 American In stitute of Physics.