IMPROVING HARMONIC VIBRATIONAL FREQUENCIES CALCULATIONS IN DENSITY-FUNCTIONAL THEORY

Using a previously introduced weight scheme, microbatching, and grid c ompression [R. E. Stratmann, G. E. Scuseria and M. J. Frisch, Chem. Ph ys. Lett. 257, 213 (1996)], we significantly speed up the numerical in tegration of the exchange-correlation contribution to the Coupled-Pert urbed Kohn-Sham equations. In addition, we find that the nature of the integrand is such that it is possible to employ substantially fewer g rid points in the quadrature and to use the Gaussian very Fast Multipo le Method (GvFMM) with very short multipole expansions for the Coulomb contribution, with negligible loss in accuracy. As a representative e xample, the computational demand for the exchange-correlation portion of a coronene (C24H12) frequency calculation with a 3-21G basis is red uced by more than one order of magnitude. The overall speed up achieve d in this calculation is between a factor of 4 to 6, depending on the specific functional. We also present sample calculations using polariz ed bases, gradient-corrected functionals, and on even larger systems ( C54H18 and C96H24), to illustrate the various effects and improvements that we have accomplished. (C) 1997 American Institute of Physics.