A critical study of some methods for evaluating the C₆, C₈ and C₁₀ isotropic dispersion energy coefficients using the first row hydrides, CO, CO₂ and N₂O as models

Abstract

Several methods currently used to calculate isotropic dispersion energy coefficients, C _2n , are investigated: (i) a version of the single excitation energy Unsöld scheme, (ii) a ‘bond’ oscillator model (which employs Frost model wavefunctions), and the generalized ab initio (iii) Unsöld and (iv) Kirkwood schemes. The large numerical discrepancies, which often occur in the results for β=C ₈/C ₆ and γ=C ₁₀/C ₆ obtained by the various methods, are ascribed to difficulties with (i) and (ii). Corrected versions of (i) and (ii) are discussed and they yield larger results for β and γ which, especially in the case of (i), are in much better agreement with those of the ab initio schemes (iii) and (iv) which show the expected marked increase in β and γ as the size of the interacting species increases. Results from the ab initio schemes, using SCF wavefunctions, are presented for C ₆, C ₈ and C ₁₀ for interactions arising from the molecules BH₃, CH₄, NH₃, H₂O, HF, CO, CO₂ and N₂O. The numerical values for the C _2n are still subject to rather large uncertainties although for some interactions the results are apparently (fortuitously) very good. The results for β and γ are much more reliable than the values of the individual C _2n and hence can provide a reasonably reliable description of isotropic long range interactions when combined with accurate values for C ₆.

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Notes on contributors

Fred Mulder

Associated with the Centre for Interdisciplinary Studies in Chemical Physics.

Gerald F. Thomas

Associated with the Centre for Interdisciplinary Studies in Chemical Physics.

William J. Meath

Associated with the Centre for Interdisciplinary Studies in Chemical Physics.