  The collective pairing hamiltonian is obtained in the framework of the
generator coordinate method in the gaussian overlap approximation with a
slightly modified BCS function used as a generator function. The collective
variable alpha, measuring the monopole moment of the pairing field, and the
gauge transformation angle phi are chosen as generator coordinates. The
vibrational ground states are calculated by diagonalisation of the collective
pairing hamiltonian in the harmonic oscillator basis.
