First-principles-based molecular dynamics simulations are reported for the (100) surface of Si. Both 21 and 42 unit cells were treated, with twelve or six layers of atoms allowed to move independently. The following results were obtained: (1) There is a strong tendency toward the formation of dimer rows. (2) Tilted dimers are preferred, and in the simulations at 200 K only tilted dimers were observed. (3) At 300 K the dimers still spend most of their time in tilted positions. However, each dimer is occasionally observed to switch among the three available geometries: tilted left, untilted, and tilted right. There are low barriers ({reversed tilde}0.1 eV) separating these local energy minima. (4) At 750 K there are large amplitude, very anharmonic vibrations about the untilted geometry. 1992.
