Properties of harmonium atoms from FCI calculations: Calibration and benchmarks for the ground state of the two-electron species.

When used in conjunction with appropriate extrapolation schemes, full configuration interaction (FCI) calculations employing systematic sequences of spherical Gaussian primitives with even-tempered exponents shared by functions of different angular momenta are capable of affording ground-state energies of the two-electron harmonium atoms with a few-muHartree accuracy that is sufficient for calibration and benchmarking of approximate electron correlation theories of quantum chemistry. The present approach, which is slated for use in future computations of electronic properties of harmonium atoms with between three and five electrons, calls for a series of 15 FCI runs involving basis sets with between four and eight Gaussian primitives of the sp, spd and spdf type. Its applicability is limited by linear dependencies among basis functions that become significant for small (i.e. less than 0.03) values of the force constant.