Spin-state ordering in metal-based compounds using the localized active space self-consistent field method

Pandharkar, R.; Hermes, M. R.; Cramer, C. J.; Gagliardi, L.
J. Phys. Chem. Lett. 2019, 10, 5507 (doi:10.1021/acs.jpclett.9b02077).

Quantitatively accurate calculations for spin-state ordering in transition-metal complexes typically demand a robust multiconfigura- tional treatment. The poor scaling of such methods with increasing size makes them impractical for large, strongly correlated systems. Density matrix embedding theory (DMET) is a fragmentation approach that can be used to specifically address this challenge. The single-determinantal bath framework of DMET is applicable in many situations, but it has been shown to perform poorly for molecules characterized by strong correlation when a multiconfigurational self-consistent field solver is used. To ameliorate this problem, the localized active space self-consistent field (LASSCF) method was recently described. In this work, LASSCF is applied to predict spin-state energetics in mono- and di-iron systems, and we show that the model offers an accuracy equivalent to that of CASSCF but at a substantially lower computational cost. Performance as a function of basis set and active space is also examined.