Harmonic vibrational frequency scaling factors for the new NDDO Hamiltonians:: RM1 and PM6

Scaling factors have been derived to obtain fundamental vibrational frequencies with the recently introduced semiempirical molecular orbital methods RM1 and PM6, implemented in MOPAC2007. A least-squares approach is used with a training set comprised of 90 singlet-state molecules and 922 distinct vibrations, extracted from the NIST Computational Chemistry Comparison and Benchmark Database (CCCBDB). Results are presented both for the conventional Scott-Radom type single-factor fitting, and for a multi- factor linear model: Semiempirical Semiglobal Self-consistently Scaled Quantum Mechanical (S4QM). The new NDDO methods in conjunction with the multi-linear fitting are shown to yield improved prediction of vibrational frequencies. To demonstrate the performance of S4QM//PM6 for calculating vibrational spectra, the examples of indene, indazole and four tetrachlorinated p-dibenzodioxins are presented.