Molecular dynamics simulations of the response to oscillating electric field elicited from an altitudinal dipolar molecular rotor mounted on the Au(111) surface and previously studied experimentally in static fields show unidirectional rotation in one of the three pairs of conformational enantiomers. The simulations are based on the universal force field and take into account electronic friction in the metal through its effect on the image charges. The rotor consists of two cobalt sandwich posts whose upper decks carry a biphenyl-like rotator with a dipole moment perpendicular to the rotation axle, mounted parallel to the surface. A phase diagram of rotor performance at 10 K as a function of field frequency and amplitude contains five unidirectional rotation regions: synchronous, half-synchronous (every other cycle skipped), quarter-synchronous (only indistinctly), asynchronous, and essentially no response. The nature of the subharmonic ''single-molecule parametric oscillator'' behavior is understood in mechanistic detail. Simulations at higher temperatures distinguish the thermal (''Brownian'') and driven regimes of rotation, elucidated in terms of time-dependent potential energy surfaces for the rotation.Brownian molecular rotors ͉ driven molecular rotors B iological systems contain functional nanoscopic machines. Modern nanotechnology has reached a level of sophistication where it seems possible to build artificial molecular machines, and rotors are among the essential parts needed (1-4). The ''Molecular Tinkertoys'' (5-8) concept has been used to prepare dipolar and nonpolar altitudinal (axle parallel to surface) molecular rotors mounted on a Au(111) surface, and barrier height imaging demonstrated that the dipolar 9,9,10,10-tetrafluoro-9,10-dihydrophenanthrene rotator in rotor 1 (Fig. 1) turns the direction of its dipole in response to a static electric field imposed by the tip of a scanning tunneling microscope (9). Under what conditions, if any, could an oscillating electric field normal to the surface be used to drive unidirectional rotation of surface-mounted 1? We use classical molecular dynamics to obtain an answer.Scanning tunneling microscope measurements showed that the rotors are firmly attached to the surface and remain in place during hours-long scan periods, but changes in the conformational arrangement of the tentacles on the scale of tens of minutes are suggested by the observed slow ''blinking'' of the differential barrier height imaging images (9). In fresh samples, rotors with ''tentacles,'' 1 (polar) and 2 (nonpolar), are believed to be attached through Au-S bonds. Because of slow oxidative cleavage of the Hg-S bond (9), in samples of 1 aged in air the attachment is through Au-Hg ϩ bonds, which also may be true when tentacle-free rotors (3; Fig. 1) are used. In our simulations, all 20 S atoms (1, 2) or all 10 Hg atoms (3) are permanently attached to fixed lattice sites of the Au(111) surface for the few nanoseconds that are being simulated. We refer to rotors 1 and 2 unless otherwise note...