We have calculated the 11S, 21S, and 23S state energies of helium atom described by the nonrelativistic Schrodinger equation, and obtained (in atomic units): E(11S) equals -2.9037243770330010, E(21S) equals -2.1459740460540720, E(23S) equals -2.1752293782367900. We have estimated the relative accuracy of the order of 10-12 minus 10-13 for the 11S and 12S states, and 10-15 minus 10-16 for the 23S state. The importance of calculations of helium for high resolution laser spectroscopy is emphasized.
E. V. Baklanov, E. V. Baklanov,
"Possibilities of high-resolution laser spectroscopy of helium", Proc. SPIE 2799, Atomic and Quantum Optics: High-Precision Measurements, (6 May 1996); doi: 10.1117/12.239850; https://doi.org/10.1117/12.239850