Theorists design new-look fullerenes.

Add some five-sided rings of carbon to a graphite sheet -- within its honeycomb arrangement of carbon atoms -- and the sheet curves around to form a buckyball or one of its rounded fullerene cousins.

But if that sheet contained seven-sided carbon rings instead of five-sided ones, an entirely different sort of fullerene would result, two independent groups of theoretical physicians report. These new theoretical molecules would be more stable than a buckyball, and they seem quite light and strong, David Vanderbilt at Rutgers University in Piscataway, N.J., and a colleague conclude in the Jan. 27 PHYSICAL REVIEW LETTERS.

Such useful properties suggest that scientists should start looking for fragments of these molecules in the soot generated during fullerene production, says Veit Elser, whose group at Cornell University studied a theoretical molecule containing 216 carbon atoms.

Vanderbilt and Jerry Tersoff at the IBM Thomas, J. Watson Research Center in Yorktown Heights, N.Y., call their new 168-carbon molecule buckygym because of its repeating, jungle-gym-like structure. Using a computer program, they constructed the buckygym by substituting seven-sided rings of carbon where fullernes typically have five-sided rings: Six-sided rings surround each seven-sided ring, and each six-sided ring is surrounded by alternating six- and seven-sided rings.

"If you use this [substitution] pattern, [the molecule] does close in on itself, but in a complex way," Tersoff says. A fullerene's 12 pentagons make the carbon sheet bend inward. But septagons cause the sheet to curl in along one axis and outward along the perpendicular axis, so parts of these sheets resemble saddles. Instead of joining to form a ball, the saddle-like segments form a network of short tubes. Molecules join and become repeating units in a lattice arrangement. The arrangement is tatrahedral, like the structure of diamond, and leads to a structure that looks nothing like a buckyball's, he adds.

"It's sort of a lacy network, with all this empty space," says Tersoff. "Yet even though it is mostly vacuum, it's still macroscopically rigid." Thus, the teams expect a material made with these molecules to be quite strong, but lightweight. Also, these molecules seem more stable than buckyballs. The seven-sided rings allow carbons to bond with one another with less strain than do pentagons, note both research groups. Vanderbilt and Tersoff calculate that the buckygym needs about one-sixth the energy per atom that the buckyball needs to form.

Working with Michael Teter at Corning Inc., in Corning, N.Y., Elser and his group decided to add extra hexagons as well as replace the pentagons with septagons, thereby making it easier for repeating molecules to link up with less strain. In their report in the Jan. 23 NATURE, they call the substance schwarzite. They also studied theoretical molecules with eight-carbon rings.

"The big question is the feasibility of synthesizing these things," says Elser. "We're in the same position that the buckyball people were in five or six years ago." Both groups suspect, however, that parts of these molecules may form during the production of fullerenes. Slow-growing sheets curl inward to make buckyballs, while the fastest-growing sheets of carbon may take on these saddle curves, Elser adds.

To help chemists recognize these new molecules when they see them, Vanderbilt and Tersoff plan to calculate buckygym's electronic structure and vibration modes, characteristics that would signal the existence of saddle-shaped carbon sheets.