Configuration interaction benchmark for Be ground state

A set of 432 energy-optimized Slater-type radial orbitals together with spherical harmonics up to a = 30 is used to approximate the corresponding full configuration interaction (CI) expansion for Be ground state. An analysis of radial and angular patterns of convergence for the energy yields a basis set incompleteness error of 8.7 mu hartree of which 85% comes from radial basis truncations for a a parts per thousand currency sign 30. Select-divide-and-conquer CI (Bunge in J Chem Phys 125:014107, 2006; Bunge and Carb-Dorca in J Chem Phys 125:014108, 2006) produces an energy upper bound 0.02(1) mu hartree above the full CI limit. The energy upper bound E = -14.6673473 corrected with these two truncation energy errors yields E = -14.6673560 a.u. (Be) in fair agreement with the latest explicitly correlated Gaussian results of E = -14.66735646 a.u. (Be). The new methods employed are discussed. It is acknowledged that at this level of accuracy traditional atomic CI has reached a point of diminishing returns. Modifications of conventional (orbital) CI to seek for significantly higher accuracy without altering a strict one-electron orbital formalism are proposed.