An application of chemometric techniques to analyze the effects of wave function modifications on the binding energies of hydrogen-bonded complexes

Factorial design and principal component models are used to determine how ab initio H-bond energies depend on characteristics of the molecular orbital wave functions of HCN-HX linear complexes and acetylene-HX T-shaped complexes, with X = F, NC, Cl, CN and CCH. The results obtained for the two sets of complexes show that factorial design and principal component analyses complement each other. The H-bond energies of the HCN-HX linear complexes are affected mostly by adding polarization functions and by MP2 treatment, which have opposite signs. When polarization functions are introduced in the basis set the calculated H-bond energies decrease by approximate to 4 kJ.mol(-1). In contrast, when the level of calculation is changed from Hartree-Fock to MP2, the energies are increased by nearly the same amount on average. The principal component analysis shows that for the HCN-HX linear complexes the ab initio results are grouped into four well-separated classes: (I) HF calculations with polarization functions (HF/6-nG** and HF/6-n+ +G**), with n = 31 or 311; (II) MP2 calculations without polarization functions (MP2/6-nG and MP2/6-n+ +G); (III) MP2 calculations including polarization functions (MP2/6-nG** and MP2/6-n+ +G**) and (IV) HF results without polarization functions (HF/6-nG and HT/6-n+ +G). The ab initio results of the group (III) are those that show a better agreement with the available experimental values. The change from the Hartree-Fock to the MP2 level is the most significant effect for the acetylene-HX complexes, increasing the calculated H-bond energy values by about 4.5 kJ.mol(-1) on average. Here the ab initio results can be essentially grouped into two classes: HF or MP2 calculations. The H-bond energies calculated from the factorial models for the two sets of complexes are found to deviate only 6.5% from the fall ab initio values. (C) 2002 Elsevier Science B.V. All rights reserved.