The CCSD(T)-MP2 term combined with MP2/CBS is used across computational chemistry as a benchmark, or used in place where one cannot afford an extensive series of CCSD(T) computations to extrapolate to CBS. FNO-CCSD(T) produces 4 different MP2 energies. One in the full basis, one for the FNO basis, one for NAF basis, and then another one before the FNO-DF-CCSD iterations start. Which MP2 energy is the correct one to obtain the MP2 in CCSD(T) - MP2?
