Abstract
Rigorous lower and upper bounds on the lowest three eigenvalues of the two-electron atomic Hamiltonian are obtained for the symmetry sectors 1 S, 3 S, 1 P, 3 P and the sector 3 P o with unnatural parity. The bounds result from three-dimensional projection operators and are given as explicit expressions that depend on the nuclear charge Z as parameter. They are designed for application within further analysis, and we exemplify this by demonstrating monotonicity properties of excitation energies.
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