Electronic and vibrational properties of C60 at finite temperature from ab initio molecular dynamics

Atomistic versus two-body central potential models of C(60): a comparative molecular dynamics study

We report on an extensive molecular dynamics investigation of two models of C60. The first model is based on an effective pair, central potential obtained by integrating the interaction between two carbon atoms over the fullerene cages [L.A. Girifalco, J. Phys. Chem. 96, 858 (1992)]. The second model explicitly takes into account the discrete, "atomistic" structure of the C60 molecules; we study two different parametrizations of the carbon-carbon interaction, one identical to that employed in the Girifalco approach, the other borrowed from previous studies on graphite [A. Cheng and M.L. Klein, J. Phys. Chem. 95, 6750 (1991)]. We consider a temperature range spanning from 300 to 1900 K, and pressures up to 200 kbar. Results for the lattice spacing and several thermodynamic quantities, as well as for the radial distribution functions, are reported and compared among each other and with experimental data. The central pair model yields only semiquantitative predictions at typical ambient densities, whereas pressures are generally overestimated. Atomistic simulations reproduce to an overall quantitative level of accuracy the experimental C60 properties. A comparison is also made of the central versus the atomistic potential predictions, when using the same potential parameters in the carbon-carbon interaction. We discuss applications of the adopted modelizations to fullerene systems of current interest, as well as different strategies to optimize the values of the potential parameters.

Intermolecular interactions and the nature of orientational ordering in the solid fullerenes C 60 and C 70

We have proposed an intermolecular potential for C 60 molecules that not only reproduces the correct low-temperature structure, but also correlates a wide range of experimental properties, including the molecular reorientational time in the room-temperature rotator phase, the volume change at the orientational ordering transition, and the librational frequencies in the low-temperature phase. The low- pressure phases in solid C 70 have been explored using constant-pressure molecular dynamics and an intermolecular potential derived from one that gives an excellent account of the properties of solid C 60 . The molecular dynamics calculations predict three low-pressure phases: a high-temperature rotator phase, a partly ordered phase with trigonal symmetry, and an ordered monoclinic phase. The calculations on C 70 were carried out on a cluster of IBM RS/6000s, operating in parallel.

Structural and Electronic Properties of La@C 82

The structural and electronic properties of the La@C(82) fullerene have been investigated by means of the Car-Parrinello method, which is based on the local density approximation of the density functional theory. The topological arrangement of the C(82) cage was assumed to be a C(3v) symmetry isomer. Three configurations were considered, one with the lanthanum atom at the center of the cluster, one with it along the threefold axis, and one with it at a low-symmetry, highly coordinated site. The structure was fully relaxed and it was found that the last of these configurations is energetically preferred. In this position, the lanthanum atom is nearly in a La(3+) state and the unpaired electron is somewhat delocalized on the cage, in agreement with available experimental data. This arrangement suggests that the chemical shifts of the 5s and 5p lanthanum states can be used as a structural probe and as a way of further validating this picture. It is argued that this conclusion is not affected by the assumed fullerene structure.