Cooperative Interplay of van der Waals Forces and Quantum Nuclear Effects on Adsorption: H at Graphene and at Coronene

Abstract

The energetic barriers that atoms and molecules often experience when binding to surfaces are incredibly important to a myriad of chemical and physical processes. However these barriers are difficult to describe accurately with current computer simulation approaches. Two prominent contemporary challenges faced by simulation are the role of van der Waals forces and nuclear quantum effects. Here we examine the widely studied model systems of hydrogen

Hydrogenation Facilitates Proton Transfer through Two-Dimensional Honeycomb Crystals

“…Analysis reveals that this reduction in the free energy barrier is due to enhanced quantum delocalisation of the proton at the transition state compared to the initial state. This is similar behavior to that observed for H chemisorption on graphene [24], and is illustrated by the snapshots shown in Fig. 2.…”

Simulation and understanding of atomic and molecular quantum crystals

“…Consequently, hydrogenation of large areas of graphene could be achieved more easily in practice than previously inferred from classical simulation studies. Interestingly, Davidson et al (2014) have pointed to the need of explicitly considering van der Waals forces in this type of quantum simulation studies; the estimated energetic barriers for the chemisorption and diffusion of H atoms then are reduced further, in some cases as much as ∼ 25 % (depending on the employed DFT functional). In view of the results presented in the last part of this section, we can conclude that inclusion of QNE and long-range dispersive interactions in modeling of hydrogenated carbon-based nanomaterials is necessary for providing a realistic estimation of gas-adsorption capacities and transition states at low temperatures.…”

Quantum dynamics of hydrogen atoms on graphene. I. System-bath modeling

“…On one hand, most DFT periodic calculations in the Generalized Gradient Approximation (GGA) give a barrier of ∼0.2 eVa value which is considerably reduced when van der Waals corrected functionals are employed. 23 On the other hand, accurate wavefunctions calculations on cluster models by Wang et al 14 suggest that GGA functionals overestimate the binding energy, hence underestimate the barrier (by more than 0.2 eV, see Ref. 14).…”

“…9,26 Finally, despite the apparent simplicity of the system, building a dynamical model which is suited to study the process in the low collision energy regime remains a challenging problem. Many models have been proposed in the past, highlighting that many different effects need to be simultaneously accounted for to reach a quantitative description of the process: 23,[26][27][28][29][30][31][32] (i) due to the fast substrate relaxation induced by the sp 2 -sp 3 conversion, forces on the binding carbon atom are large and the motion of the latter is strongly coupled to the hydrogen coordinate; 27 (ii) a large fraction of the reaction takes place at the non-collinear geometries, since steering of the projectile is operative; 28 (iii) energy relaxation to graphene phonons is a relatively fast process and large amounts of energy need to be transferred such that saturation effects are likely when truncating the phonon basis; 26,32 and (iv) quantum effects have large consequences on the sticking probability, particularly at the low incident energies of interest for the chemistry of the ISM where tunneling dominates. 23,28,30,31 In the present work, we devise a model for hydrogen chemisorption that takes into account all the requirements listed above and use it in a fully quantum study of the sticking dynamics.…”

“…Among these developments, noteworthy is the formulation of accurate vdW-DFT functionals which overcome some of the limitations of the semilocal functionals, 34,35 namely, the inability to describe nonlocal dispersion forces which have been shown recently to affect both the physisorption well and the sticking barrier in the present problem. 23 Furthermore, the increasing feasibility of direct, ab initio molecular dynamics approaches allows one to bypass the need of developing lattice potentials and modeling the system-environment coupling when describing energy transfer to the surface.…”