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Dispersion Interaction Between Two Atoms or Molecules

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Part of the SpringerBriefs in Molecular Science book series (BRIEFSMOLECULAR)

Abstract

In this chapter, diagrammatic time-dependent perturbation theory is employed to calculate the Casimir-Polder dispersion potential between two neutral electric dipole polarisable atoms or molecules. Its computation via the minimal-coupling scheme is summarised first. Next, it is shown how the energy shift may be computed more simply by adopting the multipolar Hamiltonian in the electric dipole approximation. In this second framework the force is mediated by the exchange of two virtual photons, and the Casimir-Polder formula results on summing the contribution from twenty-four time-ordered diagrams evaluated at fourth-order of perturbation theory. The potential is obtained for oriented as well as for isotropic systems separated beyond the region of wave function overlap. Asymptotically limiting forms of the interaction energy applicable in the near-and far-zone regions are also found. The former reproduces the London dispersion formula, while the latter exhibits an inverse seventh power law due to the effects of retardation.

Keywords

Diagrammatic perturbation theory Casimir-Polder potential London dispersion energy Vacuum electromagnetic field Fluctuating electric dipoles 

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© The Author(s)  2016

Authors and Affiliations

  1. 1.Department of ChemistryWake Forest UniversityWinston-SalemUSA

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