Convergence of the CCSD(T) Correction Term for the Stacked Complex Methyl Adenine-Methyl Thymine: Comparison with Lower-Cost Alternatives.

We have performed large-scale calculations for the interaction energy of the stacked methyl adenine-methyl thymine complex at the CCSD(T)/aug-ccpVXZ (X = D,T) levels. The results can serve as benchmarks for the evaluation of two methods, MP2.5, introduced recently, and the widely used ΔCCSD(T) correction defined as the difference between the CCSD(T) and MP2 energies. Our results confirm that the ΔCCSD(T) correction converges much faster toward the complete basis set (CBS) limit than toward the MP2 or CCSD(T) energies. This justifies approximating the CBS energy by adding the ΔCCSD(T) correction calculated with a modest basis set to a large basis MP2 energy. The fast convergence of the ΔCCSD(T) correction is not obvious, as the individual CCSD and (T) contributions converge less rapidly than their sum. The MP2.5 method performs very well for this system, with results very close to CCSD(T). It is conjectured that using a ΔMP2.5 correction, defined analogously to ΔCCSD(T), with large basis sets may yield more reliable nonbonded interaction energies than using ΔCCSD(T) with a smaller basis set. This would result in important computational savings as the MP3 scales computationally much less steep than CCSD(T), although higher than SCS-MP2, a similar approximation.