Using density functional theory and Monte Carlo calculations, we study the thickness dependence of the magnetic and electronic properties of a van der Waals interlayer antiferromagnet in the two-dimensional limit. Considering ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ as a model material, we find it to demonstrate a remarkable set of thickness-dependent magnetic and topological transitions. While a single septuple layer block of ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ is a topologically trivial ferromagnet, the thicker films made of an odd (even) number of blocks are uncompensated (compensated) interlayer antiferromagnets, which show wide band gap quantum anomalous Hall (zero plateau quantum anomalous Hall) states. Thus, ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ is the first stoichiometric material predicted to realize the zero plateau quantum anomalous Hall state intrinsically. This state has been theoretically shown to host the exotic axion insulator phase.

We study the dynamics of transient hot H atoms on Pd(100) that originated from dissociative adsorption of ${\mathrm{H}}_{2}$. The methodology developed here, denoted AIMDEF, consists of ab initio molecular dynamics simulations that include a friction force to account for the energy transfer to the electronic system. We find that the excitation of electron-hole pairs is the main channel for energy dissipation, which happens at a rate that is five times faster than energy transfer into Pd lattice motion. Our results show that electronic excitations may constitute the dominant dissipation channel in the relaxation of hot atoms on surfaces.

The relaxation dynamics of hot H, N, and ${\mathrm{N}}_2$ on Pd(100), Ag(111), and Fe(110), respectively, is studied by means of ab initio molecular dynamics with electronic friction. This method is adapted here to account for the electron density changes caused by lattice vibrations, thus treating on an equal footing both electron-hole ($e-h$) pair and phonon excitations. We find that even if the latter increasingly dominate the heavier is the hot species, the contribution of $e-h$ pairs is by no means negligible in these cases because it gains relevance at the last stage of the relaxation process. The quantitative details of energy dissipation depend on the interplay of the potential energy surface, electronic structure, and kinetic factors.

We have studied large areas of (3\ifmmode\times\else\texttimes\fi{}3)R30\ifmmode^\circ\else\textdegree\fi{} graphene commensurate with a Pt(111) substrate. A combination of experimental techniques with ab~initio density functional theory indicates that this structure is related to a reconstruction at the Pt surface, consisting of an ordered vacancy network formed in the outermost Pt layer and a graphene layer covalently bound to the Pt substrate. The formation of this reconstruction is enhanced if low temperatures and polycyclic aromatic hydrocarbons are used as molecular precursors for epitaxial growth of the graphene layers.

Experiments have shown that the efficiency of nanoporous carbons to store hydrogen becomes enhanced by doping the material with metallic nanoparticles. In particular, doping with palladium has been used with success. The hypothesis to justify the enhancement has been that the Pd nanoparticles dissociate the hydrogen molecules and then the hydrogen atoms spill over the carbon substrate, where the hydrogen is retained. To test this hypothesis we have performed ab initio molecular dynamics simulations of the deposition of molecular hydrogen on Pd nanoparticles (Pd6 and Pd13) supported on graphene, which is a good model for the wall of a carbon nanopore. Three channels have been identified in the simulations: bouncing off the molecule, molecular adsorption, and dissociation of the molecule in two H atoms. The relative percentage of those channels is sensitive to the size of the Pd particle. Dissociation occurs more frequently on Pd13 and it generally takes place on the lateral regions of the Pd particles. However, in our simulations we have not found a single case of H atoms or H2 molecules spilling over the carbon substrate. We have also tested the situation when several H atoms are preadsorbed on the Pd6 and Pd13 particles and found that not a single dissociation event occurs on these H-saturated nanoparticles. These results lead us to cast strong doubts on the validity of the spillover mechanism for explaining the enhancement of hydrogen adsorption on porous carbons doped with transition metal nanoparticles.