An empirical potential for the O-H·O Hydrogen bond: Part I. Comparison with ab initio molecular orbital results

An empirical potential energy function has been devised for the O-H·O hydrogen bond, for use with the MMI force field. The energy of the hydrogen bond is described as the sum of van der Waals, electrostatic and Morse components. The function has been used to calculate the potential energy hypersurface of the water dimer, and the results are compared with published ab initio molecular orbital studies. Satisfactory agreement is obtained except for orientations involving very short H·H contacts. The geometry and hydrogen bond energy of the equilibrium linear form of (H₂O)₂ are calculated to be r(O·O) = 2.84 Å, θ = 36°, ΔE = ?5.35 kcal mol^?1, which are close to the values obtained by experiment, and from molecular orbital calculations. The relative importance of the electrostatic component of the empirical hydrogen bond energy is consistent with molecular orbital energy decomposition studies. The empirical function has also been used to calculate the energy of the water trimer in orientations which serve as models for the crystallographic bifurcated hydrogen bond. The results indicate that, in these orientations, the trimer is typically 0–3 kcal mol^?1 more stable than the dimer, a result which is consistent with ab initio calculations.