John R. Sabin

An approximate Xα functional is proposed from which the charge density fitting equations follow variationally. LCAO Xα calculations on atomic nickel and diatomic hydrogen show the method independent of the fitting (auxiliary) bases to within 0.02 eV. Variational properties associated with both orbital and auxiliary basis set incompleteness are used to approach within 0.2 eV the Xα total energy limit for the nitrogen molecule.

The total Xα energy accurate to 0.3 eV is computed for H2, B2, C2, N2, O2, CO, and F2. Relative to experiment, the Xα model (α=0.7) is accurate to within ΔRe=0.1 bohr, ΔDe=2 eV, and Δωe=300 cm−1 for these molecules. Except for the lightest first-row diatomic molecules, the Xα and experimental dissociation energies are bracketed by those of the Hartree–Fock model (from below) and the Local Spin Density model (from above).

A new computational approach to calculating the electronic structure of thin films is presented in detail. The local-density-functional formalism in the linear combination of atomic orbitals approximation is used. The self-consistent solution of the resulting secular equations is made tractable through the use of efficient fitting procedures to approximate the charge density and its cube root. Feasibility is demonstrated by calculational results for an atomic hydrogen monolayer and an atomic beryllium monolayer. Comparison of the beryllium results with known experimental data on the beryllium surface and other calculations gives respectable agreement.

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