Effects of the basis set superposition error on optimized geometries of trimer complexes (Part I)

The basis set superposition error (BSSE) of hydrogen-bonded trimers, (NH3)3, (H2O)3, (HF)3, (PH3)3, (H2S)3 and (HCl)3, was examined using the Counterpoise (CP) approach at the MP2 level with various basis sets. Trimers from periods 1 and 2 elements showed a similar trend in the geometrical variations, even though the CP-corrections on the interaction energies of the latter were much larger. The degree of the changes in the bond length was not significant when a sufficiently large basis set was used. The CP-correction function, δE(fCP), made a considerable contribution to the complexation energy, ΔEc, even with the largest basis set.