Two-electron wavefunctions. Fourteenth-order expectation values

Variational perturbation theory is used to calculate a 14-order ground-state wavefunction for two electrons moving about a nucleus of charge Z. Fractional exponents are used to approximate the logarithmic terms in the exact wavefunction. Perturbation expansion coefficients for 15 operators are then calculated. Estimates of the accuracy of the calculation are obtained by comparison with exactly known first-order expectation values and with expectation values recovered from the perturbation energies of Baker, Freund, Hill and Morgan (Phys. Rev. A 41 (1990) 1247). (C) 1998 Published by Elsevier Science B.V. All rights reserved.