Molecules are nothing like horses. But if they were, they'd be a bit smarter, because they would have figured out that trotting is a colossal waste of energy. Researchers placed four different carbon-ring molecules with oxygen atoms for "legs" on a copper surface. Heat exchange propelled them forward. Two-legged molecules moved roughly the way humans walk: one leg after the other. The four-legged ones walked in much the same way, moving first the atoms on one side, then the atoms on the other—a gait known as pacing. They never trotted like a horse does. That's because, according to simulations the team reports  online this month in the Journal of the American Chemical Society, moving diagonally opposite legs at the same time distorts the molecule—and thus requires much more energy than pacing. But even pacing has disadvantages: The team found that two-legged molecules were much better at moving through obstacles than were four-legged ones.
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