Density Matrices and Density Functionals

pp 553-581

Extracules, Intracules, Correlation Holes, Potentials, Coefficients and All That

  • Ajit J. ThakkarAffiliated withDepartment of Chemistry, University of New Brunswick

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A brief, subjective review is given of some concepts and functions which are important to an understanding of electron pair densities in position and momentum space. Intracule (relative motion) and extracule (centre of mass) densities are three dimensional contractions of electron pair densities that retain genuine two-electron character. Study of these quantities should add to an understanding of pair densities. Coulomb and Fermi holes need to be studied in order that electron pair correlation be understood in a physical and visual manner. This can be of utility in density functional models of exchange and correlation energies. In particular, the study of correlation energy generating potentials should lead to improved correlation energy functionals. Statistical correlation coefficients provide simple, global measures of electron pair correlation. One such coefficient is particularly interesting since it can be measured experimentally.