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

The basis set dependence of higher-order correlation effects on pi-type interaction energies was examined by scanning the potential energy surfaces of five dimer systems. The dimers of acetylene (H-C equivalent to C-H), diacetylene (H-C equivalent to C-C equivalent to C-H), cyanogen (N equivalent to C-C equivalent to N), diphosphorous (P equivalent to P), and 1,4-diphosphabutadiyne (P equivalent to C-C equivalent to 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 delta(CCSD(T))(MP2), and the corresponding differences between CCSD(T) and CCSD interactions energies, denoted delta(CCSD(T))(CCSD). Double-zeta 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 delta(CCSD(T))(CCSD) 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 delta(CCSD(T))(MP2) term. While triple-zeta basis sets typically improved results, only aug-cc-pVTZ provided appreciable improvement over utilizing the aug-cc-pVDZ basis set to compute delta(CCSD(T))(CCSD). Counterpoise (CP) corrections were also applied to all double- and triple-zeta 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 delta(CCSD(T))(MP2), 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. (C) 2012 American Institute of Physics. [doi: 10.1063/1.3671950]