The role of anharmonicity in single-molecule spin-crossover

We exploit the system-bath paradigm to investigate anharmonicity effects of vibrations on spin-crossover (SCO) in a single molecule. Focusing on weak coupling, we use the linear response approximation to deal with the vibrational bath and propagate the Redfield master equation to obtain the equilibrium high spin fraction. We take both the anharmonicity in the bath potentials and the nonlinearity in the spin-vibration coupling into account and find a strong interplay between these two effects. Further, we show that the SCO in a single molecule is always a gradual transition and the anharmonicity-induced phonon drag greatly affects the transition behavior.