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The Kramers' restricted Hartree᎐Fock KRHF and second-order Ž . Møller᎐Plesset perturbation KRMP2 methods using relativistic effective core potentials Ž . RECP with spin᎐orbit operators and two-component spinors are extended to the unrestricted forms, KUHF and KUMP2. As in the conventional unrestricted methods, the KUHF and KUMP2 methods are capable of qualitatively describing the bond breaking for a single bond. As a result, it is possible to estimate spin᎐orbit effects along the dissociation curve at the HF and MP2 levels of theory as is demonstrated by the test calculations on the ground states of HI and CH I. Since the energy lowering due to 3 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.

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