Breaking bonds with the state-selective multireference coupled-cluster method employing the single-reference formalism

The state-selective (SS) multireference coupled-cluster (CC) method exploiting the single-reference (SR) formalism [P. Piecuch, N. Oliphant, and L. Adamowicz, J. Chem. Phys. 99, 1875 (1993)] is applied to BH and H₂O at equilibrium and displaced geometries. Different selections of active spaces are considered. Comparison with the SR CC approaches involving single and double (CCSD), single, double, and triple (CCSDT) and single, double, triple, and quadruple (CCSDTQ) excitations, and with the full configuration interaction method, indicates remarkable stability and accuracy of the SS CC results for difficult bond breaking cases. For the first time, the ab initio SS CC calculations include the most complete SS CCSD(TQ) approximation, which emerges through selection of the most essential clusters appearing in the full SR CCSDTQ theory.