Dimer switching on Si(100)

First-principles-based molecular dynamics simulations are reported for the (100) surface of Si. Both 2×1 and 4×2 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 (∽0.1 eV) separating these local energy minima. (4) At 750 K there are large amplitude, very anharmonic vibrations about the untilted geometry.