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Register a new userIntroducing Open boundary conditions in modeling nonperiodic materials and interfaces: the impact of the periodic assumption
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Simulations are essential to accelerate the discovery of new materials and to gain full understanding of known ones. Although hard to realize experimentally, periodic boundary conditions are omnipresent in material simulations. In this work, we intro-duce ROBIN (recursive open boundary and interfaces), the first method allowing open boundary conditions in material and interface modeling. The computational costs are limited to solving quantum properties in a focus area which allows explicitly discretizing millions of atoms in real space and to consider virtually any type of environment (be it periodic, regular, or ran-dom). The impact of the periodicity assumption is assessed in detail with silicon dopants in graphene. Graphene was con-firmed to produce a band gap with periodic substitution of 3% carbon with silicon in agreement with published periodic boundary condition calculations. Instead, 3% randomly distributed silicon in graphene only shifts the energy spectrum. The predicted shift agrees quantitatively with published experimental data. Key insight of this assessment is, assuming periodici-ty elevates a small perturbation of a periodic cell into a strong impact on the material property prediction. Periodic boundary conditions can be applied on truly periodic systems only. More general systems should apply an open boundary method for reliable predictions.
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Continuum solvation methods can provide an accurate and inexpensive embedding of quantum simulations in liquid or complex dielectric environments. Notwithstanding a long history and manifold applications to isolated systems in open boundary conditions, their extension to materials simulations --- typically entailing periodic-boundary conditions --- is very recent, and special care is needed to address correctly the electrostatic terms. We discuss here how periodic-boundary corrections developed for systems in vacuum should be modified to take into account solvent effects, using as a general framework the self-consistent continuum solvation model developed within plane-wave density-functional theory [O. Andreussi et al. J. Chem. Phys. 136, 064102 (2012)]. A comprehensive discussion of real-space and reciprocal-space corrective approaches is presented, together with an assessment of their ability to remove electrostatic interactions between periodic replicas. Numerical results for zero-dimensional and two-dimensional charged systems highlight the effectiveness of the different suggestions, and underline the importance of a proper treatement of electrostatic interactions in first-principles studies of charged systems in solution.
An algorithm implemented in an open-source python library was developed for building periodic coincidence site lattice (CSL) grain boundary models in a universal fashion. The software framework aims to generate tilt and twist grain boundaries from cubic and tetragonal crystals for ab-initio and classical atomistic simulation. This framework has two useful features: i) it can calculate all the CSL matrices for generating CSL from a given Sigma ({Sigma}) value and rotation axis, allowing the users to build the specific CSL and grain boundary models; ii) it provides a convenient command line tool to enable high-throughput generation of tilt and twist grain boundaries by assigning an input crystal structure, {Sigma} value, rotation axis, and grain boundary plane. The developed algorithm in the open-source python library is expected to facilitate studies of grain boundary in materials science. The software framework is available on the website: aimsgb.org.
We report a significant Dzyaloshinskii-Moriya interaction (DMI) and perpendicular magnetic anisotropy (PMA) at interfaces comprising hexagonal boron nitride (h-BN) and Co. By comparing the behavior of these phenomena at graphene/Co and h-BN/Co interfaces, it is found that the DMI in latter increases as a function of Co thickness and beyond three monolayers stabilizes with one order of magnitude larger values compared to those at graphene/Co, where the DMI shows opposite decreasing behavior. At the same time, the PMA for both systems shows similar trends with larger values for graphene/Co and no significant variations for all thickness ranges of Co. Furthermore, using micromagnetic simulations we demonstrate that such significant DMI and PMA values remaining stable over large range of Co thickness give rise to formation of skyrmions with small applied external fields in the range of 200-250 mT up to 100 K temperatures. These findings open up further possibilities towards integrating two-dimensional (2D) materials in spin-orbitronics devices.
In order to model a spiral spin state in a thin film, we study a classical Heisenberg model with open boundary conditions. With magnetic field applied in the plane of the film, the spin state becomes ferromagnetic above a critical field that increases with thickness $N$. For a given $N$, the spiral passes through states with $n= n_0$ up to 0 complete periods in steps of 1. These numerical results agree with earlier analytic results in the continuum limit and help explain the susceptibility jumps observed in thin films.
Net atomic charges (NACs) are widely used in all chemical sciences to concisely summarize key information about the partitioning of electrons among atoms in materials. Although widely used, there is currently no atomic population analysis method suitable for being used as a default method in quantum chemistry programs. To address this challenge, we introduce a new atoms-in-materials method with the following nine properties: (1) exactly one electron distribution is assigned to each atom, (2) core electrons are assigned to the correct host atom, (3) NACs are formally independent of the basis set type because they are functionals of the total electron distribution, (4) the assigned atomic electron distributions give an efficiently converging polyatomic multipole expansion, (5) the assigned NACs usually follow Pauling scale electronegativity trends, (6) NACs for a particular element have good transferability among different conformations that are equivalently bonded, (7) the assigned NACs are chemically consistent with the assigned atomic spin moments, (8) the method has predictably rapid and robust convergence to a unique solution, and (9) the computational cost of charge partitioning scales linearly with increasing system size. Across a broad range of material types, the DDEC6 NACs reproduced electron transfer trends, core electron binding energy shift trends, and electrostatic potentials across multiple system conformations with excellent accuracy compared to other charge assignment methods. Due to non-nuclear attractors, Baders quantum chemical topology could not assign NACs for some of these materials. The DDEC6 method alleviates the bifurcation or runaway charges problem exhibited by earlier DDEC variants and the Iterative Hirshfeld method. These characteristics make the DDEC6 method ideally suited for use as a default charge assignment method in quantum chemistry programs.
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