Fourier-Legendre expansion of the one-electron density-matrix of ground-state two-electron atoms

TL;DR

This paper introduces a Fourier-Legendre expansion method for the one-electron density matrix in two-electron atoms, demonstrating rapid convergence and capturing correlation effects with minimal terms, enhancing understanding of electron correlation and angular delocalization.

Contribution

The paper derives closed-form expressions for the Fourier-Legendre series coefficients of the density matrix and shows that a two-term expansion effectively captures correlation effects in two-electron systems.

Findings

Two-term expansion accounts for over 99% of electrons and kinetic energy.

Series converges rapidly with respect to electron number and kinetic energy.

Correlation affects angular delocalization of electrons.

Abstract

The density-matrix rho(r, r') of a spherically symmetric system can be expanded as a Fourier-Legendre series of Legendre polynomials Pl(cos(theta) = r.r'/rr'). Application is here made to harmonically trapped electron pairs (i.e. Moshinsky's and Hooke's atoms), for which exact wavefunctions are known, and to the helium atom, using a near-exact wavefunction. In the present approach, generic closed form expressions are derived for the series coefficients of rho(r, r'). The series expansions are shown to converge rapidly in each case, with respect to both the electron number and the kinetic energy. In practice, a two-term expansion accounts for most of the correlation effects, so that the correlated density-matrices of the atoms at issue are essentially a linear functions of P1(cos(theta)) = cos(theta). For example, in the case of the Hooke's atom: a two-term expansion takes in 99.9 % of the electrons and 99.6 % of the kinetic energy. The correlated density-matrices obtained are finally compared to their determinantal counterparts, using a simplified representation of the density-matrix rho(r, r'), suggested by the Legendre expansion. Interestingly, two-particle correlation is shown to impact the angular delocalization of each electron, in the one-particle space spanned by the r and r' variables.