Arabidopsis thaliana UV RESISTANCE LOCUS8 (UVR8) is a photoreceptor protein that mediates light responses to Ultavilet (UV)-B (280-315 nm) of the solar spectrum in plant. Upon UV-B irradiation, the UVR8 dimers will spontaneously dissociate into monomers. We have developed a multistate density functional theory to model the excited-state electron transfer process and the subsequent dimer dissociation. We found that the photochemically induced charge transfer processes lead to increased water content at the dimer interface, causing dissociation of the UVT8 homodimer. In this talk, we first introduce multistate density functional theory, and then, we present results on computation of the dynamic processes of the electron transfer reactions and a plausible mechanism for the photodissociation.