Dissociation of methane into hydrocarbons at extreme (planetary) pressure and temperature

Science

Volumn 275, Issue 5304, 1997, Pages 1288-1290

  Ancilotto, F ^a,b   Chiarotti, G L ^b,c   Scandolo, S ^b,c,d   Tosatti, E ^b,c,d  

^a UNIVERSITY OF PADOVA   (Italy)

^b INFM   (Italy)

^c SISSA   (Italy)

^d ABDUS SALAM INTERNATIONAL CENTRE FOR THEORETICAL PHYSICS   (Italy)

Author keywords

[No Author keywords available]

Indexed keywords

HYDROCARBON; METHANE;

ARTICLE; ASTRONOMY; COMPUTER SIMULATION; MOLECULAR DYNAMICS; PRESSURE; PRIORITY JOURNAL; THERMODYNAMICS;

ATMOSPHERE; BUTANES; COMPUTER SIMULATION; ETHANE; EXTRATERRESTRIAL ENVIRONMENT; HYDROCARBONS; HYDROGEN; METHANE; NEPTUNE; PRESSURE; TEMPERATURE; THERMODYNAMICS; URANUS;

EID: 0031055530     PISSN: 00368075     EISSN: None     Source Type: Journal    
DOI: 10.1126/science.275.5304.1288     Document Type: Article

References (29)

1. W. B. Hubbard and J. M. MacFarlane, J. Geophys. Res. 85, 225 (1980).
2. W. B. Hubbard et al., Science 253, 648 (1991).
3. R. V. Eck, E. R. Lippincott, M. O. Dayhoff, Y. T. Pratt, ibid. 153, 628 (1966).
4. W. J. Nellis, F. H. Ree, M. Van Thiel, A. C. Mitchell, J. Chem. Phys. 75, 3055 (1981).
5. M. Ross, Nature 292, 435 (1981).
6. W. G. Hoover, Computational Statistical Mechanics (Elsevier, Amsterdam, 1991).
7. P. Focher, G. L. Chiarotti, M. Bernasconi, E. Tosatti, M. Parrinello, Europhys. Lett. 26, 345 (1994); S. Scandolo, M. Bernasconi, G. L. Chiarotti, P. Focher, E. Tosatti, Phys. Rev. Lett. 74, 4015 (1995).
8. P. Focher, G. L. Chiarotti, M. Bernasconi, E. Tosatti, M. Parrinello, Europhys. Lett. 26, 345 (1994); S. Scandolo, M. Bernasconi, G. L. Chiarotti, P. Focher, E. Tosatti, Phys. Rev. Lett. 74, 4015 (1995).
9. The simulation cell is allowed to vary in shape and volume, driven by the quantum mechanically calculated internal stress (21). The ion-electron interaction is modeled with a fully nonlocal pseudopotential for carbon and a local pseudopotential for hydrogen. Electron-electron interactions are treated within the local density approximation, with gradient corrections as described in (22). A plane-wave basis set with a kinetic energy cutoff of 40 rydberg has been used for the electronic wave functions, which are assumed to have the same periodicity as the (varying) simulation cell.
10. 4 equation of state at 20 GPa and 2000 K.
11. The time scale of our simulations is such that a few hundred C-H stretching oscillations and a few tens of molecular collisions take place in each run. Although this is typically a sufficient time scale for thermalization, and in fact dissociation took place in a fraction of a picosecond, processes like segregation and diffusion occur on a longer time scale. These processes are energetically minor in comparison with dissociation and will not significantly alter the dissociation picture as proposed.
12. W. J. Nellis, F. H. Ree, R. J. Trainor, A. C. Mitchell, M. B. Boslough, J. Chem. Phys. 80, 2789 (1984).
13. W. J. Nellis et al., Science 240, 779 (1988).
14. M. S. Somayazulu, L. W. Finger, R. J. Hemley, H. K. Mao, ibid. 271, 1400 (1996).
15. F. H. Ree, J. Chem. Phys. 70, 974 (1979).
16. Mixtures, of course, do not represent a well-defined state at T = 0. Their enthalpy is an upper bound to that of the true low-temperature system, where single phases are likely to prevail.
17. 4 molecule.
18. 4 dissociation in forthcoming low-temperature diamond-anvil cell experiments.
19. J. I. Moses, K. Rages, J. B. Pollack, Icarus 113, 232 (1995).
20. B. Bezard, P. N. Romani, B. J. Conrath, W. C. Maguire, J. Geophys. Res. 96, 18961 (1991).
21. 2 do not withstand the pressure conditions of the middle ice layers (23).
22. P. Focher and G. L. Chiarotti, in Progress in Computational Physics of Matter, L. Reatto and F. Manghi, Eds. (World Scientific, Singapore, 1995), pp. 1-42.
23. A. D. Becke, Phys. Rev. A 38, 3098 (1988); J. P. Perdew, Phys. Rev. B 33, 8822 (1986).
24. A. D. Becke, Phys. Rev. A 38, 3098 (1988); J. P. Perdew, Phys. Rev. B 33, 8822 (1986).
25. M. Bernasconi, M. Parrinello, G. L. Chiarotti, P. Focher, E. Tosatti, in Proceedings of the Joint XV AIRAPT & XXXIII EHPRG International Conference on High Pressure Science & Technology, W. A. Trzeciakowski, Ed. (World Scientific, Singapore, 1996), p. 846.
26. S. Scandolo, G. L. Chiarotti, E. Tosatti, Phys. Rev. B 53, 5051 (1996).
27. J. Kohanoff, G. L. Chiarotti, S. Scandolo, E. Tosatti, in preparation.
28. 4 and hydrocarbon mixtures were obtained from low-temperature simulations. Results for diamond and hydrogen were obtained from standard total-energy calculations carried out separately, with the same level of accuracy, for carbon (24) and for hydrogen (25).
29. We acknowledge useful discussions with J. Kohanoff. The work at SISSA was partly supported by the European Commission and the Italian Research Council.