  We present a second-order N-electron valence state perturbation theory
(NEVPT2) based on a density matrix renormalization group (DMRG) reference wave
function that exploits a Cholesky decomposition of the two-electron repulsion
integrals (CD-DMRG-NEVPT2). With a parameter-free multireference perturbation
theory approach at hand, the latter allows us to efficiently describe static
and dynamic correlation in large molecular systems. We demonstrate the
applicability of CD-DMRG-NEVPT2 for spin-state energetics of spin-crossover
complexes involving calculations with more than 1000 atomic basis functions. We
first assess in a study of a heme model the accuracy of the strongly- and
partially-contracted variant of CD-DMRG-NEVPT2 before embarking on resolving a
controversy about the spin ground state of a cobalt tropocoronand complex.
