Simulations were carried out on 138-molecule clusters freezing isothermally at 130, 120, and 80 K. At 120 K, the nucleation rate was the same as in our prior simulations performed adiabatically but the final product was different. During the nanosecond period of the runs, clusters transforming adiabatically had frozen to bcc crystals while warming from 120 K to about 130 K. On the other hand, isothermal clusters at 120 and 130 K changed to monoclinic clusters after passing through the bcc phase. Clusters cooled to 80 K froze to a variety of structures. The number of molecules whose Voronoi polyhedra qualified them as being in bcc embryos grew in size erratically, and in most runs it was difficult to use the Voronoi information by itself to identify a well-characterized nucleation time. Therefore, a more discriminating; criterion for the onset of nucleation was devised. The 138-molecule clusters proved to be too small to yield definitive profiles of the several order parameters characterizing the change from the liquid phase to the critical nucleus. Even though the sizes of the nuclei were not established accurately, it was clear that critical nuclei were considerably larger than the five-molecule size forecast by the classical theory of homogeneous nucleation. At the deep supercooling of the simulations, precritical and critical nuclei were extremely ramified and haphazard in molecular orientation, but the chaotically organized nuclei at 120 and 130 K quickly annealed and grew to single crystals in most clusters. Clues were found suggesting that surface molecules may participate in the formation of critical nuclei, contrary to our long-standing belief. From nucleation rates were derived the kinetic parameters sigma(sl), the solid-liquid interfacial free energy of the classical nucleation theory, and delta, the interface thickness of Granasy's diffuse interface theory (DIT). In addition, the effect of pressure on the DIT, a new treatment of errors, and an improved weighted least-squares analysis of nucleation data were developed.