We report an ab initio molecular dynamics simulation study of the solvation and dynamics of an excess electron in liquid acetonitrile (ACN).Four families of states are observed: a diffusely solvated state and three ACN core-localized states with monomer core, quasi-dimer (π*-Rydberg mode) core, and dual-core/dimer core (a coupled dual-core).These core localized states cannot be simply described as the corresponding anions because only a part of the excess electron resides in the core molecule(s).The quasi-dimer core state actually is a mixture that features cooperative excess electron capture by the π* and Rydberg orbitals of two ACNs.Well-defined dimer anion and solvated electron cavity were not observed in the S-10 ps simulations, which may be attributed to slow dynamics of the formation of the dimer anion and difficulty of the formation of a cavity in such a fluxional medium.All of the above observed states have near-IR absorptions and thus can be regarded as the solvated electron states but with different structures, which can interpret the experimentally observed IR band.These states undergo continuous conversions via a combination of long-lasting breathing oscillation and core switching, characterized by highly cooperative oscillations of the electron cloud volume and vertical detachment energy.The quasi-dimer core and diffusely solvated states dominate the time evolution, with the monomer core and dual-core/dimer core states occurring occasionally during the breathing and core switching processes, respectively.All these oscillations and core switchings are governed by a combination of the electron-impacted bending vibration of the core ACN molecule(s) and thermal fluctuations.