Since prior work had appeared to do this estimation successfully , we were uncertain why we were encountering such overlap problems, so we studied an even simpler system. We calculated ∆A I E M →solid of fcc argon at 4 K with 18 states as in our α-methanol free energy estimation. MBAR yielded an error estimate of infinity while TI estimated ∆A I E M →solid to be -1666.5(8) k B T which, as we show below, is incorrect. This path resulted phase space overlap diagram without overlap between the states after state number 2 (Fig. ). Apparently as the harmonic potential that holds atoms in their lattice positions tends to zero, atoms become rather mobile, dramatically decreasing phase space overlap and leading to poor free energy estimates.