Direct assessment of quantum nuclear effects on hydrogen bond strength by constrained-centroid ab initio path integral molecular dynamics

Abstract

The impact of quantum nuclear effects on hydrogen (H-) bond strength has been inferred in earlier work from bond lengths obtained from path integral molecular dynamics (PIMD) simulations. To obtain a direct quantitative assessment of such effects, we use constrained-centroid PIMD simulations to calculate the free energy changes upon breaking the H-bonds in dimers of HF and water. Comparing ab initio simulations performed using PIMD and classical nucleus molecular dynamics (MD), we find smaller dissociation free energies with the PIMD method. Specifically, at 50 K, the H-bond in (HF)(2) is about 30% weaker when quantum nuclear effects are included, while that in (H(2)O)(2) is about 15% weaker. In a complementary set of simulations, we compare unconstrained PIMD and classical nucleus MD simulations to assess the influence of quantum nuclei on the structures of these systems. We find increased heavy atom distances, indicating weakening of the H-bond consistent with that observed by direct calculation of the free energies of dissociation.

The Quantum Nature of C–H···Metal Interaction: Vibrational Spectra and Kinetic and Geometric Isotope Effects of Adsorbed Cyclohexane

“…The present isotope effects suggest that it is inevitable to take into account the quantum nature of hydrogen in the C–H···M bond, such as delocalization of the hydrogen nucleus and zero‐point energy change associated with the softening of CH bonds. More advanced theories such as path‐integral molecular dynamics simulations may shed light on the microscopic origin of the observed isotope effects.…”

Hydrogenation Facilitates Proton Transfer through Two-Dimensional Honeycomb Crystals

“…Consistent with earlier studies, a very high potential energy barrier of ∼3.6 eV is found for proton penetration of graphene via the chemisorption state. Using ab initio path-integral molecular dynamics (PIMD) [25][26][27][28][29][30][31], we take into account nuclear quantum effects (NQEs) and finite temperature thermal effects. We find that NQEs reduce the penetration barrier of graphene by 0.46 eV (12%) at 300 K, which is unlikely to be responsible for the experimentally observed high transfer rate.…”

“…The climbing image nudged elastic band (cNEB) method was used in calculating the static penetration barriers [39], with a force convergence criterion of 0.03 eVÅ −1 and all atoms were allowed to relaxed. Beyond the static description, the classical and quantum free energy profiles were obtained with constrained MD and PIMD approaches [29][30][31]; with the constraint applied on the vertical distance of the proton from the 2D layer. A 0.5 fs time step was used and the imaginary-time path in the PIMD simulations was sampled with 48 replicas, at a target temperature of 300 K. After thermalization, 30,000 steps (15 ps) were collected to calculate the constraint force, for each constraint point.…”

Hydrogen Bonding and Nuclear Quantum Effects in Clays

“…H-bonds are mediated by interactions between light nuclei, whose motion can be influenced by nuclear quantum effects (NQEs) such as zero-point energy and quantum dispersion. It is well established that in order to accurately model the structure, thermodynamics and dynamics of H-bonded systems, the quantum-mechanical nature not only of the electrons but also of the nuclear motion must be accounted for 41,[45][46][47] . Nuclear quantum effects have been shown to affect proton disordering in high-pressure portlandite 48 , and to some extent NQEs have been accounted for in clays by applying zero-point corrections to the calculated ground-state energy 49,50 .…”