ANALYTIC ENERGY GRADIENTS WITH FROZEN MOLECULAR-ORBITALS IN COUPLED-CLUSTER AND MANY-BODY PERTURBATION-THEORY METHODS - SYSTEMATIC STUDY OFTHE MAGNITUDE AND TRENDS OF THE EFFECTS OF FROZEN MOLECULAR-ORBITALS

Analytic coupled-cluster (CC) and many-body perturbation theory (MBPT) energy gradient methods with restricted Hartree-Fock (RHF), unrestric ted Hartree-Fock (UHF), restricted open-shell Hartree-Fock (ROHF), and quasi-RHF(QRHF) reference functions are extended to permit dopping co re and excited orbitals. By using the canonical property of the semica nonical ROHF orbitals and the RHF orbitals from which the QRHF referen ce function is constructed, it is shown that a general procedure can b e established not only for RHF and UHF, but also for ROHF and QRHF ref erence functions. The basic theory and implementation are reported. To provide a systematic study of the trends and magnitudes of the effect s of dropped molecular orbitals (MOs) on the structures, harmonic freq uencies, and ir intensities, we study HCN, C2H2, CO2, HO2, and C2H4 at increasing levels of correlation and basis sets. The effects of the d ropped MOs with the largest basis sets are about 0.003 Angstrom and 0. 1 degrees in structures and about 1% on harmonic frequencies and ir in tensities. The magnitude and the direction of the drop-MO effect tend to be almost constant from MBPT(2) to CCSD(T) methods. The two isomers of S-3 are studied by the drop-MO-method, yielding very accurate resu lts. (C) 1997 American Institute of Physics.