Efficient geometry optimization of molecular clusters

By combining valence coordinates (stretches, bends, and torsions) to descri be intramolecular degrees of freedom with inverse distance coordinates for intermolecular degrees of freedom, we derive an efficient set of coordinate s for the geometry optimization of molecular dusters. We illustrate the eff icacy of our new coordinates by considering randomly generated clusters of dihydrogen and water molecules. Compared to optimizations in Cartesian coor dinates, the number of cycles required for convergence is reduced by up to a factor of 30. In addition, for the dihydrogen clusters, optimizations usi ng our new cluster coordinates consistently converge to lower energy struct ures than the corresponding Cartesian optimizations. Our method is far more efficient than optimizations using Z-matrix coordinates, and it avoids all problems with near-linear bond angles that are endemic with a Z-matrix des cription of the cluster geometry. Additionally, by constraining all the int ramolecular degrees of freedom in a completely automated manner, we are abl e to carry out full rigid-body optimizations. (C) 2000 John Wiley & Sons, I nc.