Solid-state Electronic Structure Calculations Research Articles

Defect states in monolayer hexagonal BN: A comparative DFT and DFT-1/2 study

Hexagonal boron nitride (h-BN) acts like a semiconductor vacuum to point defects enabling stable and controllable spin states at room temperature which qualifies them for quantum technological applications. To characterize their properties first-principles techniques constitute indispensable tools. The currently established paradigm for such solid-state electronic structure calculations is the density functional theory (DFT). Recently its variant, so-called DFT-1/2 method was introduced with the promise of accurate band gaps without a computational overhead with respect to ordinary DFT. Here, for the monolayer h-BN we contrast DFT and DFT-1/2 results for carbon substitutional impurities (CB, CN), boron and nitrogen single vacancies (VB, VN), divacancy, and Stone-Wales defects. Comparisons with more sophisticated, yet computationally costly techniques namely, hybrid functional DFT and the GW are also made, where available. From the standpoint of defect states embedded in the band gap region we demonstrate a clear advantage of DFT-1/2 in revealing the localized states otherwise buried within either the valence or conduction band continuum due to well-known gap underestimation syndrome of the standard DFT implementations. Thus, DFT-1/2 can serve for the rapid screening of candidate defect systems before more demanding considerations.

Efficient lattice constants and energy bandgaps for condensed systems from a meta-GGA level screened range-separated hybrid functional.

A meta-generalized gradient approximation (meta-GGA) level screened hybrid functional is developed for the solid-state electronic structure calculations. Assessment of the proposed functional for the solid-state lattice constants and bandgaps indicates that it is quite efficient in describing those properties. Specifically, the improvement in the bandgap performance of the presently proposed meta-GGA level screened hybrid functional is noticeable. From the construction point of view, the present screened hybrid functional is one step forward to the density functional screened hybrid functional rung by adding extra ingredients in its functional form. The most appealing feature of the present screened functional is that it is constructed upon an accurate semilocal functional by adopting a simple modification on the top of that functional.

Electronic and magnetic structure of neutral radical FBBO

The fluorinated oxobenzo-bridged bisdithiazolyl radical FBBO was recently observed to undergo a pressure-induced Mott insulator-to-metal transition, suggesting a novel organic system for studying Mott physics. This report describes the electronic structure of this material in relation to the observed magnetic response at low pressures. Through analysis of antiferromagnetic resonance measurements, we identify a layered antiferromagnetic ordered phase below ${T}_{N}=13$ K at ambient pressure, which requires strong ferromagnetic coupling between nearest neighbours. The origin of such coupling is elucidated from both molecular and solid-state electronic-structure calculations, which suggest a minimal two-orbital model with strong Hund's-rule coupling. This layered phase is partially frustrated by a second-nearest-neighbor antiferromagnetic coupling, which drives a magnetic phase transition at elevated pressure. On the basis of the two-orbital model, we suggest the pressure-induced Mott transition to proceed via rehybridization of the frontier molecular orbitals, resulting in a half-filled insulator to quarter-filled metal crossover.

Embedded correlated wavefunction schemes: theory and applications.

Conspectus Ab initio modeling of matter has become a pillar of chemical research: with ever-increasing computational power, simulations can be used to accurately predict, for example, chemical reaction rates, electronic and mechanical properties of materials, and dynamical properties of liquids. Many competing quantum mechanical methods have been developed over the years that vary in computational cost, accuracy, and scalability: density functional theory (DFT), the workhorse of solid-state electronic structure calculations, features a good compromise between accuracy and speed. However, approximate exchange-correlation functionals limit DFT's ability to treat certain phenomena or states of matter, such as charge-transfer processes or strongly correlated materials. Furthermore, conventional DFT is purely a ground-state theory: electronic excitations are beyond its scope. Excitations in molecules are routinely calculated using time-dependent DFT linear response; however applications to condensed matter are still limited. By contrast, many-electron wavefunction methods aim for a very accurate treatment of electronic exchange and correlation. Unfortunately, the associated computational cost renders treatment of more than a handful of heavy atoms challenging. On the other side of the accuracy spectrum, parametrized approaches like tight-binding can treat millions of atoms. In view of the different (dis-)advantages of each method, the simulation of complex systems seems to force a compromise: one is limited to the most accurate method that can still handle the problem size. For many interesting problems, however, compromise proves insufficient. A possible solution is to break up the system into manageable subsystems that may be treated by different computational methods. The interaction between subsystems may be handled by an embedding formalism. In this Account, we review embedded correlated wavefunction (CW) approaches and some applications. We first discuss our density functional embedding theory, which is formally exact. We show how to determine the embedding potential, which replaces the interaction between subsystems, at the DFT level. CW calculations are performed using a fixed embedding potential, that is, a non-self-consistent embedding scheme. We demonstrate this embedding theory for two challenging electron transfer phenomena: (1) initial oxidation of an aluminum surface and (2) hot-electron-mediated dissociation of hydrogen molecules on a gold surface. In both cases, the interaction between gas molecules and metal surfaces were treated by sophisticated CW techniques, with the remainder of the extended metal surface being treated by DFT. Our embedding approach overcomes the limitations of conventional Kohn-Sham DFT in describing charge transfer, multiconfigurational character, and excited states. From these embedding simulations, we gained important insights into fundamental processes that are crucial aspects of fuel cell catalysis (i.e., O2 reduction at metal surfaces) and plasmon-mediated photocatalysis by metal nanoparticles. Moreover, our findings agree very well with experimental observations, while offering new views into the chemistry. We finally discuss our recently formulated potential-functional embedding theory that provides a seamless, first-principles way to include back-action onto the environment from the embedded region.

The Structure of (SCN)x: A Study Using Molecular and Solid‐State Density Functional Theory Calculations

A riddle solved! Despite its simple formula, the structure of the (SCN)(x) polymer has remained elusive since its first synthesis in 1929. From energetics as well as NMR chemical shifts, based on DFT calculations, we have strong evidence that it is indeed a tangle of linear chains, made up from N-linked S(2)C(2)N five-membered rings.Molecular fragments and crystal structures based on proposed structures for polythiocyanogen were studied by using molecular and solid-state electronic structure calculations at the density functional theory level. The energetics and chemical shifts from both types of calculations indicate that a planar N-linked chain consisting of 1,2,4-dithiazole five-membered rings with adjacent rings pointing in opposite directions is the most likely local structure of the (SCN)(x) polymer.

Inelastic Neutron Scattering Spectrum of Cyclotrimethylenetrinitramine: A Comparison with Solid-State Electronic Structure Calculations

Solid-state geometry optimizations and corresponding normal-mode analysis of the widely used energetic material cyclotrimethylenetrinitramine (RDX) were performed using density functional theory with both the generalized gradient approximation (BLYP and BP functionals) and the local density approximation (PWC and VWN functionals). The structural results were found to be in good agreement with experimental neutron diffraction data and previously reported calculations based on the isolated-molecule approximation. The vibrational inelastic neutron scattering (INS) spectrum of polycrystalline RDX was measured and compared with simulated INS constructed from the solid-state calculations. The vibrational frequencies calculated from the solid-state methods had average deviations of 10 cm(-1) or less, whereas previously published frequencies based on an isolated-molecule approximation had deviations of 65 cm(-1) or less, illustrating the importance of including crystalline forces. On the basis of the calculations and analysis, it was possible to assign the normal modes and symmetries, which agree well with previous assignments. Four possible "doorway modes" were found in the energy range defined by the lattice modes, which were all found to contain fundamental contributions from rotation of the nitro groups.

Quantum behaviour of hydrogen and muonium in vacancy-containing complexes indiamond

Most solid-state electronic structure calculations are based on quantumelectrons and classical nuclei. These calculations either omit quantum zero-pointmotion and tunnelling, or estimate it in an extra step. Such quantum effectsare especially significant for light nuclei, such as the proton or its analogue,μ+. We propose a simple approach to including such quantum behaviour, in a form readilyintegrated with standard electronic structure calculations. This approach is demonstrated for anumber of vacancy-containing defect complexes in diamond. Our results suggest that for theNHV− complex, quantum motion of the proton between three equivalent potential energyminima is sufficiently rapid to time-average measurements at X-band frequencies.

Erratum: Real-space local polynomial basis for solid-state electronic-structure calculations: A finite-element approach [Phys. Rev. B59, 12 352 (1999)

Erratum: Real-space local polynomial basis for solid-state electronic-structure calculations: A finite-element approach [Phys. Rev. B<b>59</b>, 12 352 (1999)