Parallel Implementation of the Four-Component Relativistic Quasidegenerate Perturbation Theory with General Multiconfigurational Reference Functions.

A new, efficient parallel algorithm for four-component relativistic generalized multiconfigurational quasidegenerate perturbation theory (GMC-QDPT) introducing Kramers symmetry is implemented. Because it utilizes the independence of the terms in the matrix element computations, this algorithm both speeds up the calculation and reduces the computational resources required for each node. In addition, the amount of memory for two-electron integrals is reduced to three-eigths by Kramers restriction. The algorithm is applied to the d-d excitation energies of the platinum halide complexes, [PtCl4](2-), [PtBr4](2-), and [PtCl6](2-) and to the 6p-7s and 6p-7p excitation energies of the radon atom. It is shown to provide high parallelization efficiency and accurate excitation energies that agree well with experimental data.