Vibrational spectra and scaled quantum-mechanical molecular force fields

A number of questions connected with the use of scaled quantum-mechanical f orce fields in interpreting molecular vibrational spectra are considered br iefly. The Pulay method of scaling (congruent transformation of the force constant matrix) is noted to be applicable in the case where the relative accuracie s of the determination of diagonal and off-diagonal quantum-mechanical forc e constants are approximately equal. This requirement is satisfied for a qu antum-mechanical force field determined close to the Hartree-Fock Limit. Th en it is possible to carry out its correction, preserving to a maximum the features inherent to the molecule under investigation. Several examples of predicting the vibrational spectra of isotopomers, rota tional isomers, and related compounds are given. Unlike using force fields obtained by the traditional method of solving the inverse vibrational probl em and the additive scheme for transferring the force constants, scaled qua ntum-mechanical force fields make it possible to perform well-grounded theo retical vibrational analyses of all the related compounds of a molecule. (C ) 2001 Elsevier Science B.V. All rights reserved.