Martí Gimferrer

Nuclear quantum effects (NQEs) are relevant to a wide range of chemical phenomena, including photochemistry, proton-coupled electron-transfer reactions, and the description of common intermolecular hydrogen bonds. The latter may require a quantum-mechanical treatment of atomic nuclei, particularly protons. Several approaches have been developed for this purpose, many of which involve extensive potential-energy-surface calculations followed by quantum or semiclassical dynamics. However, the computational cost of these methods scales unfavourably with system size and with the number of nuclei treated quantum mechanically. Multicomponent methods provide an alternative by using wavefunction theory or density functional theory (DFT) to treat selected nuclei and electrons quantum mechanically and on an equal footing. Among these approaches, notable progress has been made within the nuclear–electronic orbital (NEO) formalism. The NEO framework offers a promising route for systematically incorporating NQEs, although its practical application has traditionally been limited by the substantial computational cost of existing algorithms. Considerable effort has therefore been devoted to developing scalable implementations, achieving speed-ups of one order of magnitude or more with almost no loss of accuracy. Moreover, NEO provides a unique framework for quantifying the influence of NQEs on the electronic density and on associated chemical-bonding descriptors, with potential applications ranging from studies of chemical reactivity to the training of machine-learning models. Here, we present recent developments in the NEO framework within our group, currently available in Molpro. Particular emphasis will be placed on evaluating how these methods describe the influence of NQEs on intra- and intermolecular interactions. Overall, our aim is to make multicomponent simulations more efficient and accessible, both in terms of computational cost and the chemical information that can be extracted from them, thereby extending the day-to-day quantum-chemistry toolbox.