Department of Chemical Engineering, University of Seoul, Seoul 130-743, South Korea.
Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716, USA.
J Chem Phys. 2014 Feb 28;140(8):084110. doi: 10.1063/1.4866451.
Accurate values of the free energies of C60 and C70 fullerene crystals are obtained using expanded ensemble method and acceptance ratio method combined with the Einstein-molecule approach. Both simulation methods, when tested for Lennard-Jones crystals, give accurate results of the free energy differing from each other in the fifth significant digit. The solid-solid phase transition temperature of C60 crystal is determined from free energy profiles, and found to be 260 K, which is in good agreement with experiment. For C70 crystal, using the potential model of Sprik et al. [Phys. Rev. Lett. 69, 1660 (1992)], low-temperature solid-solid phase transition temperature is found to be 160 K determined from the free energy profiles. Whereas this is somewhat lower than the experimental value, it is in agreement with conventional molecular simulations, which validates the methodological consistency of the present simulation method. From the calculations of the free energies of C60 and C70 crystals, we note the significance of symmetry number for crystal phase needed to properly account for the indistinguishability of orientationally disordered states.
使用扩展系综方法和接受比方法结合爱因斯坦分子模型，得到了 C60 和 C70 富勒烯晶体的自由能的精确值。当对 Lennard-Jones 晶体进行测试时，这两种模拟方法都给出了自由能的精确结果，在第五位有效数字上彼此不同。通过自由能曲线确定了 C60 晶体的固-固相变温度为 260 K，与实验结果吻合较好。对于 C70 晶体，使用 Sprik 等人的势模型[Phys. Rev. Lett. 69, 1660 (1992)]，从自由能曲线确定的低温固-固相变温度为 160 K。虽然这略低于实验值，但与传统的分子模拟一致，验证了本模拟方法的方法一致性。从 C60 和 C70 晶体自由能的计算中，我们注意到晶体相所需的对称数对于正确考虑各向同性无序态的不可区分性具有重要意义。
