Coupled-Cluster Valence-Bond Singles and Doubles (CCVB-SD) for Strongly Correlated Systems

  • CCVB is a computationally efficient approximation of the more expensive spin coupled valence bond (SCVB) model. It handles the molecules with multiple electron pairs localized in the same region, where both inter-pair and intra-pair correlations are strong.
  • CCVB is designed to deal with multiple bond breaking problems.
  • CCVB-SD successfully combines the advantages of CCVB and restricted CCSD. It describes valence correlation correctly even near multiple bond dissociation. It is spin-pure, size-extensive and also more black-box than CCVB as its energy is invariant under rotations in occupied and virtual MO space.
  • The block-tensor based implementation of CCVB-SD in Q-Chem makes large scale applications, such as calculating acene oligomers correlating up to 318 electrons in 318 orbitals, possible.
  • Applications include reaction mechanisms involving strongly correlated transition states such as pericyclic reactions.

Correlation energy error with respect to DMRG per acene unit in n-acene of different CC methods


Correlation-energy recovery for various methods in the dissociation of N2, STO-3G basis.