[en] A new constraint density functional (DFT) technique workable in combination with the projector augmented wave (PAW) and pseudoptential (PP) methods was developed. This technique calculates the effective on-site-interaction parameter, U _eff, of correlated electrons of materials, self-consistently, by using the DFT  +  U method. The U _eff determined by this technique has a clear physical meaning in that it determines the electronic structures of strongly correlated electronic systems (SCESs) and vice versa. The technique was used to determine the U _eff of correlated electrons of neodymium sesquioxide (Nd₂O₃) and iron oxide (FeO), and it was shown to be effective for this purpose. It enables first principles DFT  +  U PAW and PP calculations of SCESs free from any empirical parameters. (paper)