Phase Diagrams and Sublimation Enthalpies of Model C nG60 Fullerenes: A Comparative Study by Computer Simulation Fernando M. S. Silva Fernandes,* Filomena F. M. Freitas, and Rui P. S. Fartaria Laboratory of Molecular Simulation and CECUL, Department of Chemistry and Biochemistry, Faculty of Science, UniVersity of Lisboa, Rua Ernesto de Vasconcelos, Bloco C8, Piso 3, 1749-016 Lisboa, Portugal ReceiVed: August 19, 2002; In Final Form: October 21, 2002 A comparative study, strictly by computer simulation, of the phase diagrams and sublimation enthalpies of model C nG60 fullerenes is presented. Gibbs ensemble and Gibbs-Duhem integration Monte Carlo simulations were carried out with the effective potentials of Girifalco. The triple-point properties were determined by a direct method recently proposed by us. It is based on the behavior of the Gibbs ensemble simulations at the lowest temperature limit, and it does not involve free-energy calculations or any other theoretical approach. According to the present results, the liquid phases of the studied fullerenes (C 60 ,C 70 ,C 76 , and C 84 ) extend over ∼450 K. No sign of liquid supercooling was observed. The triple-point temperatures increase from C 60 to C 84 . This and the simultaneous effect of molecular size cause a relative dislocation of the phase diagrams to higher critical temperatures and lower densities. The simulated enthalpies of sublimation increase from C 60 to C 84 , and they are in very good agreement with the available experimental data. It is suggested that at least the predicted triple-point properties should approach those of real fullerenes. There is a strong correlation between the phase properties and the details of the interaction potentials, clearly reflected in the relative location of the phase diagram and enthalpy curves. On the whole, the simulated results are in good accordance with those recently reported by Abramo et al. (Abramo, M. C.; Caccamo, C.; Costa, D.; Pellicane, G. Europhys. Lett. 2001, 54, 468) from a combination of simulation, modified hypernetted chain (MHNC) theory, and a kind of “corresponding states” rule and confirm the consistency of the MHNC theoretical approach. The reduced properties, which also include the critical- and triple-point pressures as well as the sublimation enthalpies, confirm that some kind of corresponding states rule may be established for fullerenes. On the basis of that, the enthalpy of sublimation of C 96 is predicted. 1. Introduction Computer simulation in statistical mechanics is a powerful tool for the production of quasi-exact results for a molecular model. It serves two main purposes. On one hand, it is able to test a theoretical approach unambiguously. On the other hand, it can confirm, correct, or reject a given model against experimental evidence. Therefore, if it is possible to study a molecular model strictly by computer simulation, then we presumably get the “two sides of the coin”. This is the spirit of the present article. In a recent letter, Abramo et al. 1 reported a systematic study of the phase behavior of model fullerenes based on Girifalco’s potential (GP). 2 Monte Carlo simulations were combined with the modified hypernetted chain (MHNC) theoretical approach to obtain the vapor-liquid coexistence curve, the freezing line, and the critical- and the triple-point temperatures and densities of C 70 . Structural and dynamic properties were also calculated in the predicted liquid pocket, showing that the results are consistent with a normal liquid state. No sign of liquid supercooling was observed. From a kind of “corresponding states” rule for fullerenes, the critical- and triple-point temper- atures and densities for C 76 ,C 84 , and C 96 were predicted. They concluded that the model fullerenes have liquid phases that extend over almost 500 K, a temperature range considerably wider than the one found for C 60 (∼90 K). The mentioned temperature interval of ∼90 K for the existence of liquid C 60 seems, however, to be a matter of choice between two sets of apparently conflicting triple-point data: one, from free-energy Monte Carlo calculations, 3 that estimates a triple-point temperature considerably higher than the other based on simulation combined with the hypernetted mean spherical approximation and the modified hypernetted chain theories. 4-7 In recent articles 8,9 based on two different simulation ap- proaches to estimate the triple point and using a first-principles interaction potential (PRP) 10 and Girifalco’s potential (GP), we have shown that both interaction models for C 60 are able to reproduce two sets of triple point properties: one, in which the triple point is approached from high temperatures, agrees with free-energy-based calculations; the other, in which the triple point is approached from lower temperatures, is in accordance with the findings of HMSA and MHNC theoretical approaches. The high-temperature set indicates that the liquid phase for C 60 extends over ∼100 K, and the low-temperature one, that the liquid extends over ∼450 K. Moreover, we suggested that the high-temperature results correspond to local minima of the Gibbs free energy, whereas the low-temperature results correspond to the free-energy global minima. Then, from the low-temperature triple points, we simulated the vapor-solid and liquid-solid * To whom correspondence should be addressed. E-mail: fsilva@fc.ul.pt.. 276 J. Phys. Chem. B 2003, 107, 276-281 10.1021/jp0267914 CCC: $25.00 © 2003 American Chemical Society Published on Web 12/03/2002