On the Importance of CP-corrected Gradient Optimization in the Study of Hydrogen Bonded Systems

Geometries, harmonic vibrational frequencies and interaction energies of the water‐hydrogen sulfide dimer, hydrogen fluoride dimer and glycine zwitterion‐water dimer were determined by the counterpoise‐corrected (CP‐corrected) gradient optimization that explicitly corrects for the basis set superposition error (BSSE) and CP‐uncorrected (normal) gradient optimization respectively at the B3LYP and MP2 levels of theory, employing the popular Pople's standard 6–31G(d), 6–31G(d, p) and 6–311 + + G(d, p) basis sets in order to assess the importance of CP‐corrected gradient optimization in the study of hydrogen bonded systems. The normal optimization of these three H‐bonded systems obtained using these popular basis sets all yielded erratic results, whereas use of CP‐corrected gradient optimization led to consistent results with those from larger basis sets. So this CP receipt becomes useful and necessary to correctly describe large systems, where the use of small basis sets may be necessary.