CCSD[T] Describes Noncovalent Interactions Better than the CCSD(T), CCSD(TQ), and CCSDT Methods.

The CCSD(T) method is often called the "gold standard" of computational chemistry, because it is one of the most accurate methods applicable to reasonably large molecules. It is particularly useful for the description of noncovalent interactions where the inclusion of triple excitations is necessary for achieving a satisfactory accuracy. While it is widely used as a benchmark, the accuracy of CCSD(T) interaction energies has not been reliably quantified yet against more accurate calculations. In this work, we compare the CCSD[T], CCSD(T), and CCSD(TQ) noniterative methods with full CCSDTQ and CCSDT(Q) calculations. We investigate various types of noncovalent complexes [hydrogen-bonded (water dimer, ammonia dimer, water ··· ammonia), dispersion-bound (methane dimer, methane ··· ammonia), and π-π stacked (ethene dimer)] using various coupled-clusters schemes up to CCSDTQ in 6-31G*(0.25), 6-31G**(0.25, 0.15), and aug-cc-pVDZ basis sets. We show that CCSDT(Q) reproduces the CCSDTQ results almost exactly and can thus serve as a benchmark in the cases where CCSDTQ calculations are not feasible. Surprisingly, the CCSD[T] method provides better agreement with the benchmark values than the other noniterative analogs, CCSD(T) and CCSD(TQ), and even than the much more expensive iterative CCSDT scheme. The CCSD[T] interaction energies differ from the benchmark data by less than 5 cal/mol on average (for all complexes and all basis sets), whereas the error of CCSD(T) is 9 cal/mol. In larger systems, the difference between these two methods can grow by as much as 0.15 kcal/mol. While this effect can be explained only as an error compensation, the CCSD[T] method certainly deserves more attention in accurate calculations of noncovalent interactions.