Transition energies and oscillator strength calculated for d-s symmetry-forbidden electronic transition for Cu+ impurities in sodium fluoride host lattice

► Embedded cluster model for impurity levels in the NaF:Cu+ system. ► Oscillator strengths (OSs) calculated by direct vibronic coupling method. ► The dependence of the OS on temperature is reported. ► OS and transition energies calculated at CASSCF and CASSCF/SOCI level.