An evaluation of water cluster geometries derived from semi-empirical AM1 calculations

Abstract

The calculated geometries of water clusters, (H₂O)_n with n = 2-6, obtained from the semi-empirical AM1 procedure differ from geometries based on ab initio calculations. The AM1 hydrogen-bonded linear dimer with C_s symmetry reported by Dewar et al., which corresponds to the most stable ab initio or experimental geometry, is only a local minimum. The AM1 C_s dimer is found to be 2.0 kcal less stable than an unusual cyclic bifurcated structure (approximately C_2h symmetry) in which one of the lone pairs of electrons from each water molecule bisects the HOH bond angle of the other molecule. An additional local minimum, 1.7 kcal more stable than the reported C_s structure, consists of a C_2v framework in which the two hydrogen atoms of one water molecule coordinate to both lone pairs of the second molecule. These results lead to questions regarding the use of the present AM1 parameterization in studies of hydrogen bonding.