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The present paper introduces a new multi-reference perturbation approachdeveloped at second order, based on a Jeziorsky-Mokhorst expansion usingindividual Slater determinants as perturbers. Thanks to this choice ofperturbers, an effective Hamiltonian may be built, allowing for the dressing ofthe Hamiltonian matrix within the reference space, assumed here to be a CAS-CI.Such a formulation accounts then for the coupling between the static anddynamic correlation effects. With our new definition of zeroth-order energies,these two approaches are strictly size-extensive provided that local orbitalsare used, as numerically illustrated here and formally demonstrated in theappendix. Also, the present formalism allows for the factorization of alldouble excitation operators, just as in internally contracted approaches,strongly reducing the computational cost of these two approaches with respectto other determinant-based perturbation theories. The accuracy of these methodshas been investigated on ground-state potential curves up to full dissociationlimits for a set of six molecules involving single, double and triple bondbreaking. The spectroscopic constants obtained with the present methods arefound to be in very good agreement with the full configuration interaction(FCI) results. As the present formalism does not use any parameter ornumerically unstable operation, the curves obtained with the two methods aresmooth all along the dissociation path.

journal of chemical physics online/the journal of chemical physics/journal of chemical physics

A Jeziorski-Monkhorst fully uncontracted Multi-Reference perturbative  treatment I: principles, second-order versions and tests on ground state  potential energy curves