Effects of counterpoise correction and basis set extrapolation on the MP2 geometries of hydrogen bonded dimers of ammonia, water, and hydrogen fluoride

Abstract

We study the combined effects of counterpoise correction and basis set extrapolation on the second-order Møller–Plesset (MP2) geometries of three hydrogen bonded dimers, namely (NH₃)₂, (H₂O)₂ and (HF)₂. For (NH₃)₂, we study three characteristic structures on its potential energy surface. In addition, we look at the basis set convergence when diffuse functions on the hydrogen atoms are left out, as well as the errors introduced by including core correlation with valence-only correlation-consistent basis sets. Overall, the counterpoise-corrected and extrapolated geometries appear to be very reliable and are in convincing agreement with the geometries from explicitly correlated MP2-F12 calculations. Obtaining geometries with errors of less than 0.001 Ångstrom and 0.5 degrees compared to the basis set limit is, however, even with these advanced methods a difficult task.