  We present a practical scheme for performing error estimates for Density
Functional Theory calculations. The approach which is based on ideas from
Bayesian statistics involves creating an ensemble of exchange-correlation
functionals by comparing with an experimental database of binding energies for
molecules and solids. Fluctuations within the ensemble can then be used to
estimate errors relative to experiment on calculated quantities like binding
energies, bond lengths, and vibrational frequencies. It is demonstrated that
the error bars on energy differences may vary by orders of magnitude for
different systems in good agreement with existing experience.
