Ground-state energy of any atom

An extremely simplified version of the atomic shell model is used to obtain an explicit parameter-free approximation to the ground-state energy Esub(tot)(Z) of any atom. The resultant values of Esub(tot)(Z) agree to within a few per cent with those obtained from Hartree-Fock calculations for any Z in the Z = 2 to Z = 102, the agreement being of the order of 0.5% or better for Z > 45. This is an order of magnitude better than Thomas-Fermi (TF) theory in this range. For Z → infinity, where TF theory becomes exact (Lieb-Simon theorem), it is found that /r-1sub(j)> → 2/7, which is the exact TF value, and Esub(tot) approximately -CZsup(7/3) with C = 0.74, which is within 4% of Csub(exact) = 0.77. (author)