Effects of wave function modifications on calculated carbon-carbontriple bond lengths

Two-level factorial design (FD) and principal component (PC) models are used to determine the effects of wave function modifications on calculated carbon-carbon triple bond lengths for the HCCH, HCCF, HCCCH3, CH3CCCH3 and HCCCCH molecules. Our results have shown that valence, diffuse and polarization main effects are significant at the three levels of calculation here employed: Hartree-Fock (HF), Møller-Plesset 2 (MP2) and density functional theory (DFT) with B3LYP exchange-correlation functional. The valence-polarization interaction effect is significant at only the HF and MP2 levels of calculation. When valence and polarization functions are introduced in the basis set, the calculated CC bond length values decrease, in contrast with the results obtained when diffuse functions are used. The latter increase the CC bond length by +0.0016 Å, at all levels of calculations. Changing the method from HF to B3LYP and from B3LYP to MP2 yields an increase (averaged values) of +0.0195 and +0.0169 Å, respectively, on the calculated CC bond length values. For each level of calculation it was possible to establish an algebraic model to explain how the calculated CC bond length values depend on the molecular orbital wave functions. Such algebraic models successfully reproduce calculated CC bond lengths for the HCCCN and HCCCl molecules, which were not included in our training set. Our FD and PC models lead to the B3LYP/6-31G(d,p) and B3LYP/6-311++G wave functions as the selected ones in order to better reproduce experimental CC bond lengths for all the seven molecules here studied (training and validation molecules). Their predicted values deviate by just 0.004 Å from the experimental ones.