  Basis set extrapolations are typically rationalized either from analytical
arguments involving the partial-wave or principal expansions of the correlation
energy in helium-like systems, or from fitting extrapolation parameters to
reference energetics for a small(ish) training set. Seeking to avoid both, we
explore a third alternative: extracting extrapolation parameters from the
requirement that the BSSE (basis set superposition error) should vanish at the
complete basis set limit. We find this to be a viable approach provided that
the underlying basis sets are not too small and reasonably well balanced. For
basis sets not augmented by diffuse functions, BSSE minimization and energy
fitting yield quite similar parameters.
