A perturbative correction to the quadratic coupled-cluster doubles method for higher excitations

A perturbative correction to the quadratic coupled-cluster doubles (QCCD) method is proposed. This correction, QCCD(2), is based on modifying the existing second-order correction to optimized-orbital coupled-cluster doubles to avoid double-counting contributions from quadruple excitations. Comparisons against full configuration interaction calculations are presented for the equilibrium bond distance and harmonic vibrational frequency of the nitrogen molecule and for the dissociation of the nitrogen and water molecules in the cc-pVDZ basis set.