Density Matrix Renormalization Group Pair-Density Functional Theory\n (DMRG-PDFT): Singlet-Triplet Gaps in Polyacenes and Polyacetylenes

The density matrix renormalization group (DMRG) is a powerful method to treat\nstatic correlation. Here we present an inexpensive way to add additional\ndynamic correlation energy to a DMRG self-consistent field (DMRG) wave function\nusing pair-density functional theory (PDFT). We applied this new approach,\ncalled DMRG-PDFT, to study singlet-triplet gaps in polyacenes and\npolyacetylenes that require active spaces larger than the feasibility limit of\nthe conventional complete active-space self-consistent field (CASSCF) method.\nThe results match reasonably well the most reliable literature values and have\nonly a moderate dependence on the compression of the initial DMRG wave\nfunction. Furthermore, DMRG-PDFT is significantly less expensive than other\ncommonly applied ways of adding additional correlation to DMRG, such as DMRG\nfollowed by multireference perturbation theory or multireference configuration\ninteraction.\n