Molecular dynamics simulation for an ensemble of coupled quantum two level systems with classical degrees of freedom

An ensemble of model particles represented as quantum two level systems with induced dipole–dipole‐type interactions is studied with molecular dynamics techniques. The particles are treated as oriented molecules according to the direction of the actual dipole moment. A set of equations of motion is derived on the basis of a time‐dependent Hartree method, considering the classical limit for the rotational motion. From the numerical calculations, three different types of behavior for the ensemble of model particles are found. For weak interactions the system nearly behaves like an ensemble of uncoupled particles. With increasing interaction strength, the intramolecular energy starts to oscillate between the two levels, while in the strong interaction range the quantum states of individual molecules are represented by stationary linear combinations of the two states of the model quantum system for the particles. The change in behavior, when going from medium to strong interaction, occurs like a phase transition in bulk matter. The simple model presented here is well suited to describe the generic properties of an ensemble of molecules which may perform an intramolecular rearrangement which is essentially quantum mechanical. Similarities of the model system to a set of ammonia molecules (with the well‐known inversion) are pointed out.