be predicted by both pure electronic structure calculations and multicomponent CNEO-DFT calculations (Supporting Information, Figure ). In H 3 O - 2 , the proton is shared by two OH - anions, which still leads to a symmetric structure. However, the geometry optimization result for H 3 O - 2 strongly depends on whether the shared proton is treated classically or quantum mechanically. Pure electronic structure calculations give a double-well ground state PES, and if the proton is considered to be a classical particle, it will locate closer to one of the OH -, essentially forming H 2 O • OH -(Supporting Information, Figure ). In contrast, if path-integral simulations are performed based on the double-well potential, the quantum treatment of the proton brings the zero-point energy to the system, which is higher than the proton transfer barrier, resulting in a proton density centered in the middle. With CNEO-DFT, no matter which electronic functional is used, because the proton is treated quantum mechanically together with electrons, the proton is consistently predicted to be at the center and equally shared by the two OH -anions (Supporting Information, Figure ).