CC theory involves a series of approximations labeled CCSD, CCSD(T), CCSDT, etc., where 'S' stands for single excitations, 'D' for double excitations, and 'T' for triple excitations. For instance, CCSD includes single and double excitations, while CCSD(T) adds a perturbative treatment of triple excitations. These hierarchical methods allow for systematic improvement and better accuracy.
