The difference between CCSD(T) and MP2 converges quickly with increase in basis functions and hence a smaller basis set can be used for this part of calculation. Psi4 performs these calculations in stages and the treatment follows this equation: E CBS total = F scf scheme E scf basis total,SCF + F corl scheme E corl basis corl,corl wfn + δ delta wfn delta wfn lesser (1) where, F is an energy or energy extrapolation scheme. And, in our case this translates to