Vibrational Spectroscopy, 1 (1990) 159-165 Elsevier Science Publishers B.V., Amsterdam 159 zyxwvutsr Ab initio prediction of vibrational spectra: a database approach Peter Pulay *, Geza Fogarasi ‘, Xuefeng Zhou and Patterson W. Taylor Department of Chemistry, University of Arknnsas, Fayetteville, AR 7.2701 (U.S.A.) (Received 20th April 1990) Efforts to develop a database of quadratic force fields for organic molecules are described. The database is based on systematic ab initio calculations, scaled to reproduce the experimentally observed frequencies. The choice of the theoretical method, the basis sets, geometries, internal coordinates and the scaling procedure are discussed. A key point in the procedure is the automatic generation of the internal valence coordinates. This is also very advantageous for geometry optimization. The database should permit the prediction of vibrational frequencies for most organic molecules to lo-20 cm-‘, together with semiquantitative intensities. The accuracy is sufficient to identify unknown compounds from a list of reasonable candidates. Keywords: Vibrational spectra; Ab initio prediction The price/ performance ratio of small and medium-sized computers has improved by almost two orders of magnitude in the past 5 years 2. There is no sign that this trend will cease in the near future, and it is very likely that computers capable of executing at today’s supercomputer speeds, around 100 Mflops, will be available by 1995 at typical workstation prices of ca. $100000. This technical development makes the present computational project possible; it would have been impossible a few years ago. The ab initio calculation of vibrational force fields is well developed, as has been reviewed [l-3]. For calculations on larger systems, the Hartree-Fock approximation with moderate basis r Permanent address: Laboratory of Theoretical Chemistry, E%tvos L. University, Budapest, Hungary. 2 e.g., the Apollo DNlOOOO workstation can accommodate four processors, each delivering about 10 Mflops (million Boating point operations per second) for a scalar product of a few thousand elements; even higher speed has been re- ported for the R6CGOseries workstations from IBM. sets, ranging from double-zeta (DZ) to triple-zeta plus polarization (TZ + P) quality, is still the only possibility, although second-order Moller-Plesset force constants [4] are on the verge of being routinely applicable. Force constants calculated at the Hartree-Fock (HF) level have been remarka- bly successful in reproducing quadratic force fields, particularly the sign and relative magnitude of coupling (off-diagonal) terms. However, ab ini- tio calculations at this level still have significant systematic errors both for the force constants and for geometries. These errors usually make the the- oretical frequencies larger than the experimental values, although part of the discrepancy is due to the anharmonicity of the molecular vibrations. Because of the systematic errors, essentially every ab initio prediction of vibrational spectra in larger molecules includes an empirical scaling pro- cedure [5-91. A number of recent publications have used a crude scaling with a single scale factor for all frequencies. This simple procedure can be performed directly in Cartesian coordinates, ob- 0924-2031/9il/$O3.50 0 1990 - Elsevier Science Publishers B.V.