Hexagonal boron nitride (hBN) has recently emerged as a fascinating platform for room-temperature quantum photonics due to the discovery of robust visible light single-photon emitters. In order to utilize these emitters, it is necessary to have a clear understanding of their atomic structure and the associated excitation processes that give rise to this single photon emission. Here, we performed density-functional theory (DFT) and constrained DFT calculations for a range of hBN point defects in order to identify potential emission candidates. By applying a number of criteria on the electronic structure of the ground state and the atomic structure of the excited states of the considered defects, and then calculating the Huang-Rhys (HR) factor, we found that the CV defect, in which a carbon atom substitutes a boron atom and the opposite nitrogen atom is removed, is a potential emission source with a HR factor of 1.66, in good agreement with the experimental HR factor. We calculated the photoluminescence (PL) line shape for this defect and found that it reproduces a number of key features in the experimental PL lineshape.

We present a density functional theory investigation of the interaction between water and cerium oxide surfaces, considering both the stoichiometric and the reduced surfaces. We study the atomic structure and energetics of various configurations of water adsorption (for a water coverage of 0.25 ML) and account for the effect of temperature and pressure of the environment, containing both oxygen and water vapor, employing the ab initio atomistic thermodynamics approach. Through our investigation, we obtain the phase diagram of the water-ceria system, which enables us to discuss the stability of various surface structures as a function of the ambient conditions. For the stoichiometric surface, we find that the most stable configuration for water is when it is bonded at the cerium site, involving two O-H bonds of hydrogen and oxygen atoms at the surface. If oxygen vacancies are introduced at the surface, which is predicted under more reducing conditions, the binding energy of water is stronger, indicating an effective attractive interaction between water molecules and oxygen vacancies. Water dissociation, and the associated activation energies, are studied, and the role of oxygen vacancies is found to be crucial to stabilize the dissociated fragments. We present a detailed analysis of the stability of the water-ceria system as a function of the ambient conditions, and focus on two important surface processes: water adsorption/desorption on the stoichiometric surface and oxygen vacancy formation in the presence of water vapor. A study of the vibrational contribution to the free energy allows us to estimate the effect of this term on the stability range of adsorbed water.

We present density functional theory investigations of the bulk properties of cerium oxides ͑CeO 2 and Ce 2 O 3 ͒ and the three low index surfaces of CeO 2 , namely, ͑100͒, ͑110͒, and ͑111͒. For the surfaces, we consider various terminations including surface defects. Using the approach of "ab initio atomistic thermodynamics," we find that the most stable surface structure considered is the stoichiometric ͑111͒ surface under "oxygen-rich" conditions, while for a more reducing environment, the same ͑111͒ surface, but with subsurface oxygen vacancies, is found to be the most stable one, and for a highly reducing environment, the ͑111͒ Ce-terminated surface becomes energetically favored. Interestingly, this latter surface exhibits a significant reconstruction in that it becomes oxygen terminated and the upper layers resemble the Ce 2 O 3 ͑0001͒ surface. This structure could represent a precursor to the phase transition of CeO 2 to Ce 2 O 3 .