In this work, the geometric and electronic structure of the neutral and charged nitrogen–phosphorus defects were for the first time rigorously investigated by density functional theory. The ground state structures were located by screening all possible geometric configurations of the defects. It is shown that the modified self-interstitial nitrogen–phosphorus defect passivate the free carrier states of isolated substitutional phosphorus, known to be an excellent dopant in crystalline silicon. Furthermore, the band gap is shown to be similar in magnitude to bulk silicon, but direct. However, this study indicate that the nitrogen–phosphorus defect is possibly less stable than the self-interstitial nitrogen dimer at high nitrogen defect concentrations. Finally, the vibrational spectra were analyzed by means of linear response theory and phonon calculations. The resulting vibrational spectra yield a peak split of the modified self-interstitial nitrogen mode of 4 THz compared to the isolated self-interstitial nitrogen defect.