Erroneous behaviour of the widely used MP2(full)/aug-cc-pVXZ (X = D,T) level of theory for evaluating the BSSE in ion–π complexes

Several complexes of benzene with alkaline cations and s-triazine with halide anions have been optimized using the MP2 method and several double-ζ and triple-ζ basis sets. The BSSE (basis set superposition error) counterpoise correction on the ion–π complexes was computed for Pople’s and Dunning’s basis sets using both frozen core and full core MP2 methods. An anomalous behaviour of the widely used MP2(full)/aug-cc-pVXZ (X = D,T) level of theory was observed, specially for sodium complexes. It is solved using the MP2(full)/aug-cc-pCVXZ (X = D,T) level of theory.