Relativistic four-component multiconfigurational self-consistent-field theory for molecules: Formalism

A formalism for relativistic four-component multiconfigurational self-consistent-field calculations on molecules is presented. The formalism parallels a direct second-order restricted-step algorithm developed for nonrelativistic molecular calculations. The presentation here focuses on the differences required by the use of the Dirac Hamiltonian with the incorporation of time-reversal symmetry and point group symmetry for D2h and subgroups, providing the expressions in this framework which correspond to the nonrelativistic expressions. It is found that an efficient algorithm requires only twice the memory used by the largest nonrelativistic calculation in the equivalent basis, due to the complex arithmetic. The feasibility of the calculations is then determined more by the disk space for storage of integrals and N-particle expansion vectors.