Embedded Multireference Coupled Cluster Theory

Internally contracted multireference coupled cluster (icMRCC) theory is embedded within multireference perturbation theory (MRPT) to calculate energy differences in large strongly correlated systems. The embedding scheme is based on partitioning the orbital spaces of a complete active space self-consistent field (CASSCF) wave function, with a truncated virtual space constructed by transforming selected projected atomic orbitals (PAOs). MRPT is applied to the environment using a subtractive embedding approach that also allows for multilayer embedding. Benchmark calculations are presented for biradical bond dissociation, spin splitting in a heterocyclic carbene and hydrated Fe(II), and for the super-exchange coupling constant in solid nickel oxide. The method is further applied to two large transition metal complexes with a triple-ζ basis set: an iron complex with 175 atoms and 2939 basis functions, and a nickel complex with 231 atoms, and 4175 basis functions.