Quantum mechanical study of the nonbonded forces in water-methanol complexes.

The water-methanol dimer can adopt two possible configurations (WdM or MdW) depending on whether the water or the methanol acts as the hydrogen bond donor. The relative stability between the two configurations is less than 1 kcal/mol, and as a result, this dimer has been a challenging system to investigate using either theoretical or experimental techniques. In this paper, we present a systematic study of the dependence of the geometries, interaction energies, and harmonic frequencies on basis sets and treatment of electron correlation for the two configurations. At the highest theory level, MP2/aug-cc-pVQZ//MP2/aug-cc-pVTZ, interaction energies of -5.72 and -4.95 kcal/mol were determined for the WdM and MdW configurations, respectively, after correcting for basis set superposition error using the Boys-Bernardi counterpoise scheme. Extrapolating to the complete basis set limit resulted in interaction energies of -5.87 for WdM and -5.16 kcal/mol for MdW. The energy difference between the two configurations is larger than the majority of previously reported values, confirming that the WdM complex is preferred, in agreement with experimental observations. The effects that electron correlation have on the geometry were investigated by performing optimization at the MP2(full), MP4, and CCSD levels of theory. The approach trajectories for the formation of each dimer configuration are presented and the importance of these trajectories in the development of parameters for use in classical force fields is discussed.