Atomic many-body perturbation method based on multiconfiguration Dirac-Fock wave functions

An approach for many-body perturbation calculations of open-shell systems starting from multiconfiguration Dirac-Fock wave functions is developed. A procedure is described to construct an effective multiconfiguration potential for an optimized orthogonal complete set in the relativistic framework. Using a finite basis set, an infinite subset of many-body perturbation expansion terms, i.e., single- and pair-excitation amplitudes, is evaluated by using the coupled-cluster approach in the multireference model space spanned by configuration state functions. As an example, the calculation is performed for the correlation energy of ground-state beryllium. The convergence is compared with other methods.