Improving upon CCSD(T): LambdaCCSD(T). II. Stationary formulation and derivatives.

Analytical derivatives are formulated and implemented for the LambdaCCSD(T) method. As the historically first size-extensive and orbitally invariant extension of coupled-cluster (CC) theory to exploit the left-hand ground state eigenvector, it offers a vastly better treatment of bond breaking than does CCSD(T), and points the way toward further generalizations of single-reference CC theory that enhance its accuracy away from equilibrium geometries. Application to diatomic force curves and transition states of several well-characterized reactions are made. Surprisingly, despite LambdaCCSD(T) describing bond breaking much better than CCSD(T), for transition states both methods are extremely close in both structures and activation barriers. Force curves, however, demonstrate the overall superiority of LambdaCCSD(T).