The dynamics of photoinduced phase transition in spin-crossover materials is studied using numerical simulations of a microscopic two-variable anharmonic model, in close connection with photocrystallographic experiments on the prototype system $\left[\text{Fe}{\left(\text{btr}\right)}_2{\left(\text{NCS}\right)}_2\right].{\text{H}}_2\text{O}$. A comparative analysis of the simulated diffraction pattern with excitation duration and intensity as variables is performed. Nonlinear dynamics, threshold effect in excitation intensity and light-induced phase separation are modeled and attributed to a strong electron-lattice coupling, mediated by long-range elastic interactions. Photoinduced nucleation and domain growth of the metastable high-spin phase is evidenced and quantitatively explained in the Avrami formalism.

We analyse in detail the role of surface relaxations on the spin transition phenomenon through an Ising-like model solved in the inhomogeneous mean field approach.