where T 1 represents the excitation operator where one electron is excited from the groundstate wave function, while T 2 is the two-electron excitation operator. The CCSD(T) method considers triples electronic excitations T 3 in a perturbative manner using the CCSD wave function. CCSD(T) is known as the "gold standard" method in quantum chemistry with often less than 1 kcal mol -1 deviation from FCI or experiment when evaluating relative energies.