Electron correlation and basis set effects on strong hydrogen bond behavior: a case study of the hydrogen difluoride anion Original Research Article

A strong hydrogen bond between hydrogen fluoride and fluoride anions forms an anionic chemical species, i.e. a hydrogen difluoride anion. This paper describes the result of ab initio investigations on hydrogen bond behavior of the hydrogen difluoride anion, whose geometry and energy were evaluated at various computational levels of theory, from Hartree–Fock to QCISD(T) calculations using basis sets from cc-pVDZ to aug-cc-pV5Z. The MP4(SDQ) or QCISD level of calculations using aug-cc-pVTZ or a larger basis set suggested an almost constant value (2.27 Å) for the atom distance between two fluorine atoms. The larger basis set was utilized, the more negative (the larger) hydrogen bond energy was suggested. But when aug-cc-pVTZ or a larger basis set was utilized, the fluctuation of the calculated values for hydrogen bond energy was less than 1 kcal/mol. Accurate estimation of the hydrogen bond energy requires MP4(SDQ) geometry optimization and subsequent QCISD(T) energy estimation. But the difference in the geometry and energy between the MP2 and QCISD results was small and, thus, the MP2 results are considered reliable despite the low computational costs. The change in the potential energy was also examined for dissociation of the hydrogen difluoride anion into hydrogen fluoride and fluoride anions. The potential energy curve obtained from the computational level of QCISD(T)/aug-cc-pVTZ//MP4(SDQ)/aug-cc-pVTZ was in good agreement with that obtained from the calculation of QCISD(T)/aug-cc-pV5Z//MP4(SDQ)/aug-cc-pVTZ.