Variational incorporation of negative-energy orbitals in relativistic electronic structure calculations

Significant effects df negative-energy orbitals in molecules with light ato ms have recently been observed in calculations by perturbation theory from wave functions expressed in terms of positive-energy orbitals. We give a de tailed discussion of how to incorporate, through a rigorous variational the orem, negative-energy orbitals in relativistic calculations of bound electr onic states without any a priori knowledge of positive-energy orbitals, nei ther ad hoc potentials nor self-consistent-field (SCF) equations. The energ y contribution of negative-energy orbitals, generally shown to be of positi ve sign, can be minimized by resorting to a minimax theorem or in other mor e practical ways, thus allowing large-scale configuration interaction (CI) with (+) orbitals obtained from correlated calculations, rather than from u ncorrelated ones as in the conventional no-pair approach. General SCF equat ions are derived yielding positive-energy orbitals in the limit of the inde pendent-particle model. The method is illustrated with relativistic CI calc ulations taken from the recent Literature. (C) 2000 John Wiley & Sons, Inc.