  Quasiparticle random-phase approximation (QRPA) is applied to two nuclei, and
overlap of the QRPA excited states based on the different nuclei is calculated.
The aim is to calculate the overlap of intermediate nuclear states of the
double-beta decay. We use the like-particle QRPA after the closure
approximation is applied to the nuclear matrix elements. The overlap is
calculated rigorously by making use of the explicit equation of the QRPA ground
state. The formulation of the overlap is shown, and a test calculation is
performed. The effectiveness of the truncations used is shown.
