Basis set dependence of higher-order correlation effects in π-type interactions.

The basis set dependence of higher-order correlation effects on π-type interaction energies was examined by scanning the potential energy surfaces of five dimer systems. The dimers of acetylene (H-C≡C-H), diacetylene (H-C≡C-C≡C-H), cyanogen (N≡C-C≡N), diphosphorous (P≡P), and 1,4-diphosphabutadiyne (P≡C-C≡P) were studied in three different configurations: cross, parallel-displaced, and t-shaped. More than 800 potential energy curves (PECs) were generated by computing the interaction energies for all 15 dimer configurations over a range of intermolecular distances with the MP2, coupled-cluster single double (CCSD), and coupled-cluster single double triple (CCSD(T)) methods in conjunction with 21 basis sets ranging from a small 6-31G*(0.25) split-valence basis set to a large aug-cc-pVQZ correlation consistent basis set. Standard extrapolation techniques were also used to construct MP2, CCSD, and CCSD(T) complete basis set (CBS) limit PECs as well as CBS limit higher-order correlation corrections based on the differences between CCSD(T) and MP2 interaction energies, denoted δ(MP2)(CCSD(T)), and the corresponding differences between CCSD(T) and CCSD interactions energies, denoted δ(CCSD)(CCSD(T)). Double-ζ basis sets struggled to reproduce the former but provided quite reasonable descriptions of the latter as long as diffuse functions were included. The aug-cc-pVDZ basis deviated from the δ(CCSD)(CCSD(T)) CBS limit by only 0.06 kcal mol(-1) on average and never by more than 0.24 kcal mol(-1), whereas the corresponding deviations were approximately twice that for the δ(MP2)(CCSD(T)) term. While triple-ζ basis sets typically improved results, only aug-cc-pVTZ provided appreciable improvement over utilizing the aug-cc-pVDZ basis set to compute δ(CCSD)(CCSD(T)). Counterpoise (CP) corrections were also applied to all double- and triple-ζ basis sets, but they rarely yielded a better description of these higher-order correlation effects. CP corrections only consistently improved results when the aug-cc-pVDZ basis set was used to compute δ(MP2)(CCSD(T)), yielding mean and maximum absolute deviations from the CBS values of 0.10 and 0.39 kcal mol(-1), respectively, for all five dimer systems.