Molecular-orientational pinning on a surface: A simulated annealing study

Orientational properties of a quantum rotor interacting with a classical substrate are investigated by dynamical simulated annealing. At zero temperature a transition in the molecular orientation is observed when the molecule-substrate coupling exceeds a critical value ${\alpha }_c$. For very light molecules we find a smooth transition and a decrease in ${\alpha }_c$ with an increase in the moment of inertia I. Above a certain value of I the molecule undergoes a sharp reorientational transition that shifts to higher values of ${\alpha }_c$ with an increase in I. The observed continuous transition is always mean field in nature. At finite temperatures the molecule-substrate interaction has a significant thermal broadening effect on the reorientational transition. The rms fluctuations in the rotor energy increase dramatically while those in the order parameter exhibit a peak around ${\alpha }_c$.