Relativistic quasidegenerate perturbation theory with four-component general multiconfiguration reference functions

Relativistic quasidegenerate perturbation theory (QDPT) using general multiconfiguration (GMC) reference functions is developed and implemented. It is the relativistic counterpart of the nonrelativistic QDPT with GMC reference and thus retains all the advantages of the nonrelativistic GMC reference QDPT, such as applicability to any configuration space and small computational cost compared to the complete configuration-space case. The method is applied to the potential-energy curves of the ground states of I2 and Sb2 molecules, the excitation energies of CH3 I, and the energies of the lowest terms of C, Si, and Ge atoms, and is shown to provide a balanced description of potential-energy curves and accurate transition energies for systems containing heavy elements and to provide much better results compared to the reference function (i.e., active space configuration interaction) level.