KRAMERS UNRESTRICTED HARTREE-FOCK AND 2ND-ORDER MOLLER-PLESSET PERTURBATION-METHODS USING RELATIVISTIC EFFECTIVE CORE POTENTIALS WITH SPIN-ORBIT OPERATORS - TEST CALCULATIONS FOR HI AND CH3I

The Kramers' restricted Hartree-Fock (KRHF) and second-order Moller-Pl esset perturbation (KRMP2) methods using relativistic effective core p otentials (RECP) with spin-orbit operators and two-component spinors a re extended to the unrestricted forms, KUHF and KUMP2. As in the conve ntional unrestricted methods, the KUHF and KUMP2 methods are capable o f qualitatively describing the bond breaking for a single bond. As a r esult, it is possible to estimate spin-orbit effects along the dissoci ation curve at the HF and MP2 levels of theory as is demonstrated by t he test calculations on the ground states of HI and CH3I. Since the en ergy lowering due to spin-orbit interactions is larger for the I atom than for the closed-shell molecules, dissociation energies are reduced and bond lengths are slightly elongated by the inclusion of the spin- orbit interactions. (C) 1998 John Wiley & Sons, Inc.