  Measured cross sections for the fusion of {64}Ni with {64}Ni, {74}Ge, and
{100}Mo targets are analyzed in a coupled-channels approach. The data for the
{64}Ni target above 0.1 mb are reproduced by including couplings to the
low-lying 2^+ and 3^- states and the mutual and two-phonon excitations of these
states. The calculations become more challenging as the fusing nuclei become
softer and heavier, and excitations to multi-phonon states start to play an
increasingly important role. Thus it is necessary to include up to four-phonon
excitations in order to reproduce the data for the {64}Ni+{74}Ge system.
Similar calculations for {64}Ni+{100}Mo, and also for the symmetric
{74}Ge+{74}Ge system, show large discrepancies with the data. Possible ways to
improve the calculations are discussed.
