SELF-CONSISTENT-FIELD CALCULATIONS OF ATOMS AND IONS USING A MODIFIEDLOCAL-DENSITY APPROXIMATION

Local-density-approximation calculations of atomic structure are usefu l for the description of atoms and ions in plasmas. The large number o f different atomic configurations that exist in typical plasmas leads one to consider the expression of total energies in terms of a Taylor series in the orbital occupation numbers. Two schemes for computing th e second derivative Taylor-series coefficients are given; the second, and better one, uses the linear response method developed by Zangwill and Soven [Phys. Rev. A 21, 1561 (1980)] for the calculation of optica l response in atoms. A defect in the local-density approximation cause s some second derivatives involving Rydberg orbitals to be infinite. T his is corrected by using a modified local-density approximation that had previously been proposed [Phys. Rev. B 2, 244 (1970)].