Single-configuration descriptions of atomic ground and excited states: Ground states of He, Li, and Be

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Abstract
A formulation of the spin-coupled model (a generalized valence-bond model) for calculating wave functions based on the use of nonorthogonal orbitals is presented, permitting ground, valence excited, multiply excited, and core-valence excited states to be obtained using a first-order iterative scheme. A primary goal is to provide a simple single-configurational model combining visuality (e.g., readily yielding the one-particle density function) with accuracy, while allowing the variational minimum of the single-configuration function for a given basis set to be obtained automatically for almost any kind of state. The theory and its application to the ground-state properties of He, Li, and Be are presented here using even-tempered and Clementi-Roetti basis sets. Uniform scaling of the basis exponents leads to closely parallel energy profiles for both spin-coupled and full configuration-interaction (FCI) calculations. The difference in energies as a function of scale factor is considerably smaller than the corresponding difference between restricted Hartree-Fock and FCI calculations. Each energy profile displays a global minimum and one or more local minima, and, for the associated values of the scale factor, the calculated atomic properties show little variation in magnitude.