13.5.1 Introduction

The nuclear-electronic orbital (NEO) method¹¹⁸⁶ provides the framework for the accurate and efficient description of multicomponent systems in which more than one type of particle is treated quantum mechanically. Typically, the NEO method treats all electrons and select protons quantum mechanically on the same level with molecular orbital techniques, treating at least two nuclei classically to avoid issues associated with translations and rotations. An advantage of the NEO method is that anharmonicity, proton delocalization, and zero point energy contributions of the quantum protons are included directly in the energy calculations, geometry optimizations, reaction paths, and dynamics.