The ground state potential energy surfaces (PES’s) for diatomic nitrogen and fluorine are examined using a version of our recently published linearized multireference coupled cluster method (MR-LCCM). Comparison calculations employing a variety of standard ab initio techniques such as single reference configuration interaction singles and doubles (CISD), many-body perturbation theory (MBPT), coupled cluster single and doubles (CCSD), and multireference (MR)-CISD were also performed. In addition, the PES’s were also investigated using the newly developed CCSDT-1 method, which includes the dominant effect of T̂3. These single reference procedures fail in various ways (with the possible exception of the CCSDT-1 method), while the MR-LCCM method is shown to compare favorably to the more traditional MR-CI techniques. Like the MR-CIs, the MR-LCCM energy curves dissociate correctly and the two are nearly parallel.

A method for the efficient evaluation of nonadiabatic coupling matrix elements of the form 〈ΨI‖∂/∂RαΨJ〉 is presented. The wave functions ΨI and ΨJ are assumed to be multiconfiguration self-consistent field (MCSCF) wave functions optimized within the state averaged approximation. The method, which can treat several states simultaneously, derives its efficiency from the direct solution of the coupled perturbed state averaged MCSCF equations and the availability of other appropriate derivative integrals. An extension of this approach to SA-MCSCF/CI wave functions is described. Here it is shown that computational efficiencies can be achieved by exploiting analogies with analytic CI gradient methods. Numerical examples for C2v approach of Mg to H2 are presented.

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The electronic structure of CH 2 has been addressed using theoretical techniques which allow for the variational inclusion of large classes of excitions differing by more than two electrons from the Hartree-Fock description. Specifically., a natural orbital transformation has been used