Proton coupled electron transfer(PCET)is a key mechanism of redox reactions in many chemical and biological systems.It has been widely studied in homogeneous environments,while heterogenous PCET at interfaces has been less documented.In this talk,I will present our latest theoretical development of calculating the free energies of PCET reactions at interfaces.The model system is water oxidation at the TiO2 water interface,an extremely important chemical reaction in photocatalytic water splitting.The new computational scheme greatly simplifies the usual expensive free energy calculations,allowing for calculating PCET energies using a hybrid density functional,which is a necessity for accurate description of radical intermediates involved,at an affordable computational cost.Moreover,a novel level diagram is proposed to visualize the thermodynamics of decoupled electron and proton transfer reactions,which provides valuable insight into the thermodynamic overpotentials of PCET reactions.