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 d onor. 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 inve stigate using either theoretical or experimental techniques. In this paper, we present a systematic study of the dependence of the geometries, interac tion energies, and harmonic frequencies on basis sets and treatment of elec tron correlation for the two configurations. At the highest theory level, M P2/aug-cc-pVQZ//MP2/aug-cc-pVTZ, interaction energies of -5.72 and -4.95 kc al/mol were determined for the WdM and MdW configurations, respectively, af ter correcting for basis set superposition error using the Boys-Bernardi co unterpoise scheme. Extrapolating to the complete basis set limit resulted i n interaction energies of -5.87 for WdM and -5.16 kcal/mol for MdW. The ene rgy difference between the two configurations is larger than the majority o f previously reported values, confirming that the WdM complex is preferred, in agreement with experimental observations. The effects that electron cor relation 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 import ance of these trajectories in the development of parameters for use in clas sical force fields is discussed.