[en] A correction to nuclear structure calculations due to the finite life time of the excited states of a finite nucleus is investigated. An additional term to the Hamilton operator as well as to the wave function of a nuclear state is shown to appear. The analytical expressions of these terms are given and results of numerical calculations are discussed. The additional term to the Hamilton operator determines the life time of the decaying nuclear states. It violates charge symmetry of the nuclear forces. Its nondiagonal matrix elements describe the external mixing of the states via the continuum by which the properties of overlapping resonances are governed. Long-lived and short-lived states are described in a unified manner. The interplay between giant resonance and underlying levels is studied. The basic assumptions of the statistical nuclear reaction theories are discussed on the basis of numerical results obtained by taking into account the external mixing in a straight forward manner. The additional term to the wave function of a nuclear state describes virtual particles including clusters in the nuclear surface. It plays a role in most nuclear reactions. (author)