Multireference Moller-Plesset method with a complete active space configuration interaction reference function

A multireference Moller-Plesset (MRMP) method with a complete active space configuration interaction (CASCI) reference function has been proposed as a n accurate and computationally efficient method for treating the ground and excited states of molecules. The CASCI wave function was constructed using the self-consistent field orbitals and used as a reference function of the MRMP to incorporate the remaining dynamical correlation. The advantage in using the CASCI is that it does not require iterations, nor does it encount er convergence difficulties which may be found in complete active space sel f-consistent field (CASSCF) calculations. The scheme was applied to the pot ential curves of the ground and low-lying excited states of N-2, the potent ial curve of the ground state of CO, and the vertical valence pi-pi* excite d states of benzene. Excellent agreement between theory, experiment, and so me benchmark calculations was obtained. MRMP with CASCI is comparable in ac curacy with MRMP with CASSCF. The present scheme will open a possibility to ward accurate treatment for the ground and excited states of small to large molecular systems. (C) 2001 American Institute of Physics.