Minimal Basis Iterative Stockholder: Atoms in Molecules for Force-Field Development

Atomic partial charges appear in the Coulomb term of many force-field models and can be derived from electronic structure calculations with a myriad of atoms-in-molecules (AIM) methods. More advanced models have also been proposed, using the distributed nature of the electron cloud and atomic multipoles. In this work, an electrostatic force field is defined through a concise approximation of the electron density, for which the Coulomb interaction is trivially evaluated. This approximate "pro-density" is expanded in a minimal basis of atom-centered s-type Slater density functions, whose parameters are optimized by minimizing the Kullback-Leibler divergence of the pro-density from a reference electron density, e.g. obtained from an electronic structure calculation. The proposed method, Minimal Basis Iterative Stockholder (MBIS), is a variant of the Hirshfeld AIM method but it can also be used as a density-fitting technique. An iterative algorithm to refine the pro-density is easily implemented with a linear-scaling computational cost, enabling applications to supramolecular systems. The benefits of the MBIS method are demonstrated with systematic applications to molecular databases and extended models of condensed phases. A comparison to 14 other AIM methods shows its effectiveness when modeling electrostatic interactions. MBIS is also suitable for rescaling atomic polarizabilities in the Tkatchenko-Sheffler scheme for dispersion interactions.Comment: 61 pages, 12 figures, 2 table

Catalysis and Photocatalysis over TiO2 Surfaces Detailed from First Principles

Catalysts are involved at some stage in the manufacture process of virtually all commercially produced chemical product. Among the materials used as catalysts, metal oxides are one of the most used due to their versatility and wide range of physical properties. Identifying the principles of surface to adsorbate charge transfer is key to a better understanding of metal oxide materials as both catalysts and gas sensors. Using density functional theory (DFT), we modeled the adsorption of small molecules over stoichiometric and reduced metal oxide surfaces of group IV metals and quantify the effect of electron transfer upon adsorption. We found that charge transfer only occurs during the adsorption process of an adsorbate more electronegative than the surface. We also found a correlation between the work function of the metal oxide, and the ionic adsorption of the oxygen molecule. Mixed phase rutile/anatase catalysts show increased reactivity compared with the pure phases alone. However, the mechanism causing this effect is not fully understood. Using DFT and the +U correction we calculated the bands offsets between the phases taking into account the effect of the interface. We found rutile to have both higher conduction and valence band offsets than anatase, leading to an accumulation of electrons in the anatase phase accompanied by hole accumulation in the rutile phase. We also probed the electronic structure of our heterostructure and found a gap state caused by electrons localized in undercoordinated Ti atoms which were present within the interfacial region. Interfaces between bulk materials and between exposed surfaces both showed electron trapping at undercoordinated sites. Finally, we studied the effect of the size of gold nanoparticles in the catalytic properties of gold decorated titania surfaces. We found that the adsorption energy of several intermediates reactives in the CO oxidation and water gas shift reaction does not change with the size of the nanoparticles. In conclusion, the factor that affects the reactivity of the system is the density of undercoodinated gold atoms on the interface perimeter

Multiscale Simulations of Dynamics of Ferroelectric Domains

Ferroelectric materials exhibiting switchable polarization have been used as critical components in electronics, memory, actuators and acoustics, and electro-optics. The applications of ferroelectric materials heavily rely on the interactions between the polarization and external perturbations, such as electric field, stress, and temperature. It is therefore crucial to understand the dynamics of ferroelectric response at finite temperature. Despite the tremendous advance of computational power and the success of first-principles methods, large-scale simulations of dynamics in oxides at finite temperature can still only be performed using classical atomistic potential. We first develop a model potential based on principles of bond-valence and bond-valence vector conservation. The model potentials for PbTiO3 and BiFeO3 are parameterized using the results from first-principles calculations. The bond-valence-based force field allows for molecular dynamics simulations of ferroelectric response at large time and length scale. The intrinsic inertial response of ferroelectric domain walls is studied in PbTiO3. Examination of the evolution of the polarization and local structures of domain walls reveal that they stop moving immediately after the removal of the electric field, demonstrating that ferroelectric domain walls do not exhibit significant intrinsic inertial response. Taking the 90° domain walls in PbTiO3 as an example, we quantitatively estimate the domain wall velocity under a wide range of temperatures and electric fields. We find that many properties of ferroelectrics are dictated by the intrinsic nature of domain walls. We demonstrate that even in the absence of defects the intrinsic temperature- and field-dependence of the wall velocity can be described with a strongly non-linear creep-like region and a power-law depinning-like region. We propose a simple universal nucleation-and-growth-based analytical model that is able to quantify the dynamics of all types of domain walls in various ferroelectrics; this enables the prediction of the temperature- and frequency-dependence of coercive fields at finite temperature from first-principles. We also investigate the orientation-dependent evolution of nanoscale ferroelectric domain structures in PbZr0.2Ti0.8O3 films. Molecular dynamics simulations predict both 180° for (001)-/(101)-oriented films and 90° multi-step switching for (111)-oriented films, and these processes are subsequently observed in stroboscopic piezoresponse force microscopy. Finally, we investigate the domain walls in organometal halide perovskites. We find that organometal halide perovskites can form both charged and uncharged domain walls, due to the flexible orientational order of the organic molecules. The presence of charged domain walls will significantly reduce the band gap. We demonstrate that charged domain walls can serve as segregated channels for the diffusion of charge carriers

Suppression of phase transitions and glass phase signatures in mixed cation halide perovskites

Cation engineering provides a route to control the structure and properties of hybrid halide perovskites, which has resulted in the highest performance solar cells based on mixtures of Cs, methylammonium, and formamidinium. Here, we present a multi-technique experimental and theoretical study of structural phase transitions, structural phases and dipolar dynamics in the mixed methylammonium/dimethylammonium MA1-xDMAxPbBr3 hybrid perovskites (0 ≤ x ≤ 1). Our results demonstrate a significant suppression of the structural phase transitions, enhanced disorder and stabilization of the cubic phase even for a small amount of dimethylammonium cations. As the dimethylammonium concentration approaches the solubility limit in MAPbBr3, we observe the disappearance of the structural phase transitions and indications of a glassy dipolar phase. We also reveal a significant tunability of the dielectric permittivity upon mixing of the molecular cations that arises from frustrated electric dipoles