Towards large-scale calculations with State-Specific Multireference Coupled Cluster methods: Studies on dodecane, naphthynes, and polycarbenes

Multireference Coupled Cluster (MRCC) methods are essential for accurate descriptions of molecular systems that have nearly degenerate states. A number of these methods have been developed over the last three decades but their use has been hampered by their high computational cost. The development of highly scalable MRCC codes can significantly alleviate this problem. Our recent work on the parallelization of the Brillouin-Wigner MRCC implementation has clearly demonstrated that taking advantage of large computational resources can enable the MRCC calculations on chemically relevant molecules within practical time-to-solution and memory per core constraints. In this letter we report on a parallel implementation of the Mukherjee's state-specific MRCC model with single and double excitations (MR MkCCSD). In particular the abstraction of a methyl-group from dodecane, the singlet-triplet splitting in naphthyne isomers, and the singlet-high-spin splitting in polycarbenes are considered as demonstrations of systems with significant multireference character and of challenging size that currently can be handled with the MR MkCCSD formalism.