  The density matrix renormalization group (DMRG) is a powerful method to treat
static correlation. Here we present an inexpensive way to add additional
dynamic correlation energy to a DMRG self-consistent field (DMRG) wave function
using pair-density functional theory (PDFT). We applied this new approach,
called DMRG-PDFT, to study singlet-triplet gaps in polyacenes and
polyacetylenes that require active spaces larger than the feasibility limit of
the conventional complete active-space self-consistent field (CASSCF) method.
The results match reasonably well the most reliable literature values and have
only a moderate dependence on the compression of the initial DMRG wave
function. Furthermore, DMRG-PDFT is significantly less expensive than other
commonly applied ways of adding additional correlation to DMRG, such as DMRG
followed by multireference perturbation theory or multireference configuration
interaction.
