THE GENERATION AND USE OF DELOCALIZED INTERNAL COORDINATES IN GEOMETRY OPTIMIZATION

Following on from the earlier work of Pulay and Fogarasi [J. Chem. Phy s. 96, 2856 (1992)] we present an alternative definition of natural in ternal coordinates. This set of delocalized internal coordinates can b e generated for any molecular topology, no matter how complicated, and is fully nonredundant. Using an appropriate Schmidt-orthogonalization procedure, all standard bond length, bond angle, and dihedral angle c onstraints can be imposed within our internal coordinate scheme. Combi natorial constraints (in which sums or differences of stretches, bends , and torsions remain constant) can also be imposed. Optimizations on some fairly large systems (50-100 atoms) show that delocalized interna l coordinates are far superior to Cartesians even with reliable Hessia n information available at the starting geometry. (C) 1995 American In stitute of Physics.