  Collisions of isobar nuclei, those with the same mass number but different
structure parameters, provide a new way to probe the initial condition of the
heavy ion collisions. Using transport model simulation of $^{96}$Ru+$^{96}$Ru
and $^{96}$Zr+$^{96}$Zr collisions at two energies $\sqrt{s_{\mathrm{NN}}}=0.2$
TeV and 5.02 TeV, where $^{96}$Ru and $^{96}$Zr nuclei have significantly
different deformations and radial profiles, we identify sources of
eccentricities contributing independently to the final state harmonic flow
$v_n$. The efficacy for flow generation is different amount these sources,
which qualitatively explains the modest yet significant energy dependence of
the isobar ratios of $v_n$. Experimental measurement of these ratios at the LHC
energy and compared with those obtained at RHIC will provide useful insight
into the collision-energy dependence of the initial condition.
