Density Matrix Renormilization Group and Pair-Density Functional Theory for Molecular Excited States

Density matrix renormalization group theory has proven a highly flexible and efficient ansatz for converging molecular wave functions. However, large-scale studies of its convergence properties with respect to bond dimension and orbital character are rare. In this work, we investigate the performance of DMRG on a broad and diverse dataset of vertical excitation energies, and propose diagnostics for identifying when the DMRG ansatz has converged to an inaccurate local minima or when more orbitals need to be included in the active subspace. Additionally, we examine the convergence behavior of the DMRG excitation energy when non-classical correlation is approximated utilizing pair-density functional theory (DMRG-PDFT). Our findings show that DMRG-PDFT is able to converge to a quantitatively accurate result much more rapidly than DMRG alone, making DMRG-PDFT a promising approach for the study of strongly correlated molecular systems.