Interaction energy of the condensate in the trap versus the atom number. The solid curve is the theoretical calculation with Eq. (14). The measurements are for atom numbers of 0.55 × 10 5 , 0.66 × 10 5 , and 1.40 × 10 5 . The error bar is the uncertainty in fitting the expansion velocity. The inset shows an example for linearly fitting the measured TF radii of the condensate with the atom number of 1.40 × 10 5 .