Isotope Effects for Water at Pt(111) Computed with Nuclear−Electronic Orbital Theory

Hydrogen/deuterium (H/D) substitution at electrochemical interfaces can provide insights into fundamental electrochemical processes. Periodic nuclear–electronic orbital density functional theory (NEO-DFT), which treats specified nuclei quantum mechanically on the same level as the electrons, enables such H/D isotope effects to be investigated computationally. Herein, periodic NEO-DFT is applied to OH^–/OD^–adsorption, H/D adsorption, and H₂O/D₂O monolayers at a Pt(111) surface. These calculations inherently include anharmonic zero-point energy and nuclear delocalization of hydrogen and deuterium. Thus, they capture structural differences between H/D isotopologues, guide interpretation of experimental cyclic voltammograms, identify favored adsorption sites, and characterize differences in H₂O/D₂O hydrogen-bonding interactions. Periodic NEO-DFT maintains the favorable computational scaling of conventional DFT, predicts geometric isotope effects, and can be combined with techniques to model an applied potential. Thus, periodic NEO-DFT represents a promising tool for probing the structures of electrochemical interfaces, interpreting experimental isotope studies, and elucidating electrocatalytic mechanisms.