Variational calculation of polarizability and second hyperpolarizability of two-electron systems

Application of the perturbation expansion of the variational principle of energy in density-functional theory is demonstrated by calculating the static dipole polarizability alpha and second hyperpolarizability gamma of the two-electron systems H-, He, and Li+. Calculations are performed within both Hartree-Fock and density-functional theories. In the approach employed, energy change at each order (second and fourth in our case) is minimized so that no electric field needs to be applied explicitly. This makes the calculation very accurate. Significantly, Hartree-Fock results obtained by us, to our knowledge, are larger (and therefore closer to experiments) than those reported in the literature calculated by the finite field method, and also by other methods.