  Using the QCD sum rule with its operator product expansion reliably estimated
from lattice calculations for the pressure and energy density of the
quark-gluon plasma, we calculate the strength of the wave function of J/psi at
the origin and find that it decreases with temperature above the transition
temperature. This result is shown to follow exactly that obtained from the
solution of the Schr\"odingier equation for a charm and anticharm quark pair
using the free energy from lattice calculations as the potential and is in
sharp contrast to that using the deeper potential associated with the internal
energy, which shows an enhanced strength of the J/psi wave function at the
origin. Our result thus has resolved the long-standing question of whether the
free energy potential or the internal energy potential should be used in
analyzing the spectrum of heavy quark systems at finite temperature.
