Intermolecular Bond Lengths: Extrapolation to the Basis Set Limit on Uncorrected and BSSE-Corrected Potential Energy Hypersurfaces

Geometry optimizations were carried out for the (HF)₂, (H₂O)₂, and HF-H₂O intermolecular complexes using the MP2/aug-cc-pVXZ {X = 2, 3, 4, and 5} theoretical models on both the uncorrected and counterpoise (CP) corrected potential energy hypersurfaces (PES). Our results and the available literature data clearly show that extrapolation of intermolecular distances to the complete basis set (CBS) limit is satisfactory on PESs corrected for BSSE. On the other hand, one should avoid such extrapolations using data obtained from uncorrected PESs. Also, fixing intramolecular parameters at their experimental values could cause difficulties during the extrapolation. As the available literature data and our results clearly show, the MP2/aug-cc-pVXZ {X = 2, 3, 4} data series of intermolecular distances obtained from the CP-corrected surfaces can be safely used for the purpose of CBS extrapolations.