  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.
