Multilevel geometry optimization

Geometry optimization has been carried out for three test molecules using s ix multilevel electronic structure methods, in particular Gaussian-2, Gauss ian-3, multicoefficient G2, multicoefficient G3, and two multicoefficient c orrelation methods based on correlation-consistent basis sets. In the Gauss ian-2 and Gaussian-3 methods, various levels are added and subtracted with unit coefficients, whereas the multicoefficient Gaussian-x methods involve noninteger parameters as coefficients. The multilevel optimizations drop th e average error in the geometry (averaged over the 18 cases) by a factor of about two when compared to the single most expensive component of a given multilevel calculation, and in all 18 cases the accuracy of the atomization energy for the three test molecules improves; with an average improvement of 16.7 kcal/mol. (C) 2000 American Institute of Physics. [S0021-9606(00)30 307-5].