  We have examined the performance of the analytic Hartree-Fock-Slater (HFS)
method for various alpha (Slater's exchange parameter) values and empiricaly
determined the optimal alpha value by minimizing the mean absolute error (MAE)
in atomization energies of the G2 set of molecules. At the optimal alpha the
HFS method's performance is far superior with the
  MAE of 14 kcal/mol than that of the local density approximation (MAE ~ 36
kcal/mol) or the Hartree-Fock theory (MAE ~ 78 kcal/mol). The HFS exchange
functional with alpha = 0.7091 is found to perform significantly better than
the Kohn-Sham exchange functional for equally weighted atoms H-Kr. We speculate
that use of this alpha value may be useful in parametrization of empirical
exchange-correlation functionals.
