Harmonic force fields from scaled SCF calculations: Program ASYM40

We report an extended version of our normal coordinate program ASYM40, whic h may be used to transform Cartesian force constants from nb initio calcula tions to a force field in nonredundant internal (symmetry) coordinates. Whe n experimental data are available, scale factors for the theoretical force field may then be optimized by least-squares refinement. The alternative of refining an empirical force field to fit a wide variety of data, as with t he previous version ASYM20, has been retained. We compare the results of le ast-squares refinement of the full harmonic force field with least-squares refinement of only the scale factors for an SCF calculated force field and conclude that the latter approach may be useful for large molecules where m ore sophisticated calculations are impractical. The refinement of scale fac tors for a theoretical force field is also useful when there are only limit ed spectroscopic data. The program will accept ab initio calculated force f ields from any program that presents Cartesian force constants as output. T he program is available through Quantum Chemistry Program Exchange. (C) 200 0 Academic Press.