As such atomistic phase-field modelling has been proposed for materials science, they assume arrayed atoms in a solid state. Recently, however, phase-field modelling for a molecule state has been developed. In this modelling, a vibration of an atom from the equilibrium position is modelled as a Gaussian distribution, that is, a probability density which becomes the highest at the vibration centre. Employing an isolated density field rather than a periodic field suggests that longer time scale molecular behaviours can be investigated the same as solid states by phase-field simulations. However, in this atomistic phase-field modelling, rigid body motions, that is, diffusive motions of a molecule are still not included. Therefore, further development is needed to treat molecules in phase-field simulations.