Accurate molecular energies by extrapolation of atomic energies using an   analytic quantum mechanical model
  Using a new analytic quantum mechanical method based on Slater's Xalpha method, we show that a fairly accurate estimate of the total energy of a molecule can be obtained from the exact energies of its constituent atoms. The mean absolute error in the total energies thus determined for the G2 set of 56 molecules is about 16 kcal/mol, comparable to or better than some popular pure and hybrid density functional models. 
Rajendra R. Zope and Brett I. Dunlap (US Naval Research Laboratory,   Washington DC)