In many-electron systems, the interelectronic angle density A(theta_12) represents the probability d4nsity function for the interelectronic angle theta_ij spanned by position vectors r_i and r_j of any two electrons to be theta_12. It is shown that in general the interelectronic angle density A(theta_12) is expresed by a linear combination of Legendre polynomials P_n(cos theta_12). Explicit formulas for the expansion coefficients are presented for atoms described by determinantal wave functions. The results are applied to the 102 atoms He through Lr in their ground states within the Hartree-Fock framework, and the expansion coefficients as well as the average interelectronic angles are reported.
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