Effects of wave function modifications on calculated HC and CC stretching frequencies

Two‐level factorial design (FD) and principal component (PC) models are used to determine the effects of wave function modifications on calculated HC and CC harmonic stretching frequencies for the HCCH, HCCF, HCCCl, HCCCCH, HCCCN and HCCCH3 molecules. Our results have shown that valence (val), diffuse (dif), polarization (pol), and electron correlation (corr) main effects as well as some second‐order interaction effects are significant to build factorial models for these vibrational frequencies. For instance, when valence and diffuse functions are introduced in the basis set, the calculated HC and CC stretching frequencies decrease. However, this reduction is much more accentuated when the electron correlation effect is introduced in the Hartree‐Fock wave functions. By far, the corr main effect is the most important one on the HC and CC frequency values. The MP2 electron correlation effect provokes an increment of −157.4 cm−1 and −283.8 cm−1 on the HC and CC stretching frequencies, respectively, whereas the dif effect produces only a reduction of −9.1 cm−1 and −12.9 cm−1 on these frequencies, respectively. The inclusion of Møller–Plesset 2 perturbation in the Hartree‐Fock wave functions also produces an important pol–corr interaction effect on the HC and CC stretching frequencies, increasing their values by +45.2 cm−1 and +17.3 cm−1, respectively. Algebraic models were then established to explain how calculated HC and CC stretching frequencies depend on characteristics of the molecular orbital wave functions. These models were successful in reproducing calculated HC and CC frequency values for the HCN and CH3CCCH3 molcules, which were not included in the training set. The principal component analysis has revealed that the calculated HC and CC stretching frequencies can be adequately described by a single principal component. It is capable of explaining more than 99% of the total data variance. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008