Hylleraas-Type Calculations of the Be Ground State

Results of low-order Hylleraas-type calculations of the Be ground state are given for expansions of 1-18 terms. The 18-term expansion gives an energy of - 14.6611 a.u., which is slightly better than results of a 25-term Hylleraas-type calculation by Gentner and Burke and is comparable to the 55-term configuration-interaction result of Weiss: it is substantially poorer than results of recent much more extensive calculations by Bunge and by Sims and Hagstrom. A brief discussion is given of the additivity properties of energy contributions from expansion terms contributing to various types of correlation effects and of the relative importance, in the inner and outer shells separately, of various types of correlation effects.