Energy error bars in direct configuration interaction iteration sequence.

A computational scheme for approximate lower bound to eigenvalues of linear operators is elaborated, based on Löwdin's bracketing function. Implementation in direct full configuration interaction algorithm is presented, generating essentially just input-output increase. While strict lower bound property is lost due to approximations, test calculations result lower bounds of the same order of magnitude, as the usual upper bound, provided by the expectation value. Difference of upper and lower bounds gives an error bar, characterizing the wavefunction at the given iteration step.