Improving description of hydrogen bonds at the semiempirical level: Water-water interactions as test case

Hydrogen bonding is not well described by available semiempirical theories. This is an important restriction because hydrogen bonds represent a key fe ature in many chemical and biochemical processes, besides being responsible for the singular properties of water. In this study, we describe a possibl e solution to this problem. The basic idea is to replace the nonphysical ga ussian correction functions (GCF) appearing in the core-core repulsion term s of most MNDO-based semiempirical methods by a simple function exhibiting the correct physical behavior in the whole range of intermolecular separati on distances. The parameterized interaction function (PIF) is the sum of at om-pair contributions, each one having five adjustable parameters. In this work, the approach is used to study water-water interactions. The parameter s are optimized to reproduce a reference ab initio intermolecular energy su rface for the water-water dimer obtained at the MP2/aug-cc-pVQZ level. OO, OH, and HH parameters are reported for the PM3 method. The results of PMS-P IF calculations remarkably improve qualitatively and quantitatively those o btained at the standard PM3 level, both for water-dimer properties and for water clusters up to the hexamer. For example, the root-mean-square deviati on of the PM3-PIF interaction energies, with respect to ab initio values ob tained using 700 paints of the water dimer surface, is only 0.47 kcal/mol. This value is much smaller than that obtained using the standard PM3 method (4.2 kcal/mol). The PM3-PLF water dimer energy minimum ( 5.0 kcal/mol) is also much closer to ab initio data ( 5.0 +/- 0.01 kcal/mol) than PM3 ( 3.50 kcal/mol). The method is therefore promising for the development of new se miempirical approaches as well as for application of combined quantum mecha nics and molecular mechanicstechniques to investigate chemical processes in water. (C) 2000 John Wiley & Sons, Inc.