where ∆(T) = [E (T 1 ) -E (T 0 ) ]. This only requires the (T 1 ) iterative triples procedure to be used at the triple-ζ level, while the (T 0 ) correction is used at the quadrupleζ level. While (T 0 ) triples are more efficient than (T 1 ), they are still a significantly lengthy part of the calculation, so the second approach involves eliminating them entirely from the quadruple-ζ calculation, and correcting DLPNO-CCSD/CBS(3,4) extrapolated results with the (T 1 ) triples energy calculated at the triple-ζ level: