Plane-wave Pseudopotential Framework Research Articles

Accelerating Linear-Response Time-Dependent Hybrid Density Functional Theory with Low-Rank Decomposition Techniques in the Plane-Wave Basis.

We present an efficient low-rank implementation of linear-response time-dependent density functional theory for hybrid functionals (hybrid-LR-TDDFT) within the plane-wave pseudopotential framework. The adaptively compressed exchange (ACE) operator and the natural transition orbitals (NTOs) are introduced to build the low-rank representation of the nonlocal exchange operator in the hybrid-LR-TDDFT Hamiltonian. Numerical tests demonstrate that the ACE approximation significantly reduces the computational cost of applying the nonlocal exchange operator without loss of accuracy, and the NTO approximation can further accelerate the hybrid-LR-TDDFT calculations by introducing an NTO cutoff parameter. This new method enables us to effectively study the excitonic properties of two-dimensional MoS2 consisting of 216 atoms and ∼1900 electrons with range-separated hybrid functionals on a single graphics processing unit.

Beyond the random phase approximation with a local exchange vertex

With the aim of constructing an electronic structure approach that systematically goes beyond the GW and random phase approximation (RPA) we introduce a vertex correction based on the exact-exchange (EXX) potential of time-dependent density functional theory. The EXX vertex function is constrained to be local but is expected to capture similar physics as the Hartree-Fock vertex. With the EXX vertex we then consistently unify different beyond-RPA approaches such as the various re-summations of RPA with exchange (RPAx) and the second order screened exchange (SOSEX) approximation. The theoretical analysis is supported by numerical studies on the hydrogen dimer and the electron gas, and we discuss the importance of including the vertex correction in both the screened interaction and the self energy. Finally, we give details on our implementation within the plane-wave pseudo potential framework and demonstrate the excellent performance of the different RPAx methods in describing the energetics of hydrogen and van der Waals bonds.

First-principles study of absorption mechanism of hydrogen on W20O58 (010) surface

With the development of modern industrial technology, tungsten products prepared from traditional tungsten powder cannot meet the demands of industry. However, the properties of tungsten products produced from ultra-fine tungsten powder have been greatly improved:they have high strength, high toughness, and low metal plasticity-brittle transition temperature. Hence, it is necessary to carry out theoretical research of the micro-adsorption dynamics during hydrogen reduction of W20O58, which is beneficial to synthetizing ultra-fine tungsten powder. In this article, to comprehend the crystal characteristics of W20O58 (010) surface and provide the theoretical reaction law for hydrogen reduction on W20O58 (010) surface, the absorption mechanism of H2 molecule on W20O58 (010) surface is studied by the first-principles calculation based on density functional theory in a plane wave pseudo-potential framework. The results show that the indirect band gap of W20O58 is 0.8 eV, indicating that it has metallic characteristic. The W20O58 (010) surface has different terminations, i.e., WO-terminated (010) surface and O-terminated (010) surface. After the geometrical optimization of the two surfaces, the W–O bond length and bond angle of W–O–W are both changed. In addition, six absorption configurations of H2 on W20O58 (010) surface, including WO-L-O1c, WO-V-O1c, WO-L-O2c, WO-V-O2c, O-L-O1c and O-V-O1c, are chosen to be investigated. The calculation results show that the WO-L-O1c, WO-V-O1c and WO-L-O2c absorption system are unstable, while the WO-V-O2c, O-L-O1c and O-V-O1c absorption configuration are stable. When H2 molecule is dissociated into two H atoms, the absorption energies of the three stable configurations are-1.164 eV,-1.021 eV and-3.11 eV, respectively. It is obvious that the O-V-O1c absorption configuration is the most stable one. The analysis of density of states reveals that the 1s state of H atom interacts with the 2p and 2s states of O atom. The outermost O1c atom of O-terminated (010) surface contains an unsaturated bond, which results in the formation of bonding between two H atoms and O1c atom. As a result, an H2O molecule is formed and an oxygen vacancy on the surface is generated after absorption reaction. By combining experimental observations with simulation calculations, the mechanism of hydrogen reduction of W20O58 can be revealed from a microscopic view.

Electronic properties of surface/bulk iodine defects of CsSnBr3 perovskite

Perovskites are one of a few semiconducting materials that have been shaping the progress of the third generation photovoltaic cells. The power conversion efficiency of the perovskite solar cell has been certified as 20.1% in 2015. In this work, CsSnBr3 perovskite has been considered as an alternative material for solar cell applications because of its appropriate band gap. The study has focused on the influence of iodine impurity on the electronic properties of CsSnBr3 perovskite modeled on the (001) surface structures. The crystal structures and the electronic properties have been calculated using density functional theory (DFT) within a plane wave pseudopotential framework. The GW method has been also employed to obtain more accurate band gaps. The results show that the iodine defect improves the efficiency of photo absorbance in perovskite solar cell by lowering its energy gap. In particular, the iodine bulk defect in the SnBr2-terminated surface structure was found to exhibit the narrowest band gap. This result suggests that iodine impurity deep into the structure, e.g. iodine-dopant created from high-energy implantation, plays an important role to improve the efficiency of light absorption in CsSnBr3 perovskite.

Ab initioself-consistent total-energy calculations within the EXX/RPA formalism

Calculations of exact-exchange (EXX) and random phase approximation (RPA)-correlation energies within the formally exact adiabatic connection fluctuation-dissipation theorem formalism have recently been carried out for a number of isolated and condensed systems. Unfortunately, most of the applications have been done in a non-self-consistent procedure, and for several systems it has been found that RPA correlation energies may significantly depend on the choice of input single-particle wave functions. In this work, we develop an efficient approach to compute the EXX/RPA total energy self-consistently. We derive an expression for the RPA self-consistent potential based on the density functional perturbation theory and dielectric matrix approaches and implemented it within the plane-wave pseudopotential framework. The efficiency of this approach is greatly improved by exploiting an iterative procedure to compute the inverted Kohn-Sham density-density response function. We apply our implementation to study the binding energy curves and the structural properties of rare gasses such as Ar and Kr and alkaline-earth Be dimers. In addition, the EXX and RPA-correlation potentials of these systems at different dissociation distances are analyzed.

Cytochrome P450 compound I in the plane wave pseudopotential framework: GGA electronic and geometric structure of thiolate-ligated iron(IV)-oxo porphyrin.

The cytochromes P450 constitute a ubiquitous family of metalloenzymes, catalyzing manifold reactions of biological and synthetic importance via a thiolate-ligated iron-oxo (IV) porphyrin radical species denoted compound I (Cpd I). Experimental investigations have implicated this intermediate in a broad spectrum of biophysically interesting phenomena, further augmenting the importance of a Cpd I model system. Ab initio molecular dynamics, including Car-Parrinello and path integral methods, conjoin electronic structure theory with finite temperature simulation, affording tools most valuable to approach such enzymes. These methods are typically driven by density functional theory (DFT) in a plane-wave pseudopotential framework; however, existing studies of Cpd I have been restricted to localized Gaussian basis sets. The appropriate choice of density functional and pseudopotential for such simulations is accordingly not obvious. To remedy this situation, a systematic benchmarking of thiolate-ligated Cpd I is performed using several generalized-gradient approximation (GGA) functionals in the Martins-Troullier and Vanderbilt ultrasoft pseudopotential schemes. The resultant electronic and structural parameters are compared to localized-basis DFT calculations using GGA and hybrid density functionals. The merits and demerits of each scheme are presented in the context of reproducing existing experimental and theoretical results for Cpd I.

Nonadiabatic couplings from time-dependent density functional theory: Formulation by the Kohn-Sham derivative matrix within density functional perturbation theory

We present an improved method for accurately calculating nonadiabatic couplings (NACs) between the ground and excited states of electronic systems, using time-dependent density functional theory (TDDFT) within the plane-wave pseudopotential framework. Based on our previous work that the TDDFT formulation of NACs by the Kohn-Sham derivative matrix elements can avoid the accuracy problem caused by the nonlocal pseudopotentials, we extend the evaluation of these matrix elements to the analytical scheme, achieved by density functional perturbation theory (DFPT). As DFPT has been a standard implementation in many ab initio codes, our scheme requires little effort in the coding. Extensive calculations of NACs near various Jahn-Teller or Renner-Teller intersections show the good accuracy of our results. A comparison of our present and previous work has also clarified an important fact in the TDDFT computation of NACs, regarding the evaluation of the nuclear derivative of the many-body Hamiltonian after mapping to the Kohn-Sham system.

Plane-wave pseudopotential implementation of explicit integrators for time-dependent Kohn-Sham equations in large-scale simulations

Explicit integrators for real-time propagation of time-dependent Kohn-Sham equations are compared regarding their suitability for performing large-scale simulations. Four algorithms are implemented and assessed for both stability and accuracy within a plane-wave pseudopotential framework, employing the adiabatic approximation to the exchange-correlation functional. Simulation results for a single sodium atom and a sodium atom embedded in bulk magnesium oxide are discussed. While the first-order Euler scheme and the second-order finite-difference scheme are unstable, the fourth-order Runge-Kutta scheme is found to be conditionally stable and accurate within this framework. Excellent parallel scalability of the algorithm up to more than a thousand processors is demonstrated for a system containing hundreds of electrons, evidencing the suitability for large-scale simulations based on real-time propagation of time-dependent Kohn-Sham equations.

Calculation of NMR chemical shifts in organic solids: Accounting for motional effects

NMR chemical shifts were calculated from first principles for well defined crystalline organic solids. These density functional theory calculations were carried out within the plane-wave pseudopotential framework, in which truly extended systems are implicitly considered. The influence of motional effects was assessed by averaging over vibrational modes or over snapshots taken from ab initio molecular dynamics simulations. It is observed that the zero-point correction to chemical shifts can be significant, and that thermal effects are particularly noticeable for shielding anisotropies and for a temperature-dependent chemical shift. This study provides insight into the development of highly accurate first principles calculations of chemical shifts in solids, highlighting the role of motional effects on well defined systems.

First-principles study on effects of mechanical deformation on outer surface reactivity of carbon nanotubes

The first-principles approach of Car–Parrinello molecular dynamics is used to study the chemisorption of a single oxygen atom on the outer surface of zigzag single-wall carbon nanotubes (SWCNT) under various axial strains and bending deformation. The effect of mechanical deformation on adsorption of oxygen atom on carbon nanotubes (CNTs) is demonstrated by linking the chemical reactivity and structural deformation. The adsorption energy E b and pyramidalization angle θ P are obtained by structural relaxation calculations, and ground-state electronic structures are described according to the density functional theory (DFT) within a plane-wave pseudopotential framework. Our results show that the surface reactivity of CNT is mostly determined by its pyramidalization angle of carbon atom. For bending SWCNT, both E b and θ P vary with adsorption sites, E b is higher at sites with a larger pyramidalization angle. An approximate linear relation of strain and adsorption energy can be obtained. It is indicated that the structure of CNT is crucial for its surface reactivity, and the mechanical deformation can be used to control the surface reactivity of CNT and offer adsorption site selectivity as the adsorption is facilitated on the sites with higher pyramidalization angles.

A first principles theory of nuclear magnetic resonance J-coupling in solid-state systems

A method to calculate NMR J-coupling constants from first principles in extended systems is presented. It is based on density functional theory and is formulated within a planewave-pseudopotential framework. The all-electron properties are recovered using the projector augmented wave approach. The method is validated by comparison with existing quantum chemical calculations of solution-state systems and with experimental data. The approach has also been applied to the silicophosphate, Si(5)O(PO(4))(6), giving (31)P-(29)Si-couplings which are in excellent agreement with experiment.

Dielectric and vibrational properties of amino acids.

We calculate polarizability tensors and normal mode frequencies for the amino acids alanine, leucine, isoleucine, and valine using density functional perturbation theory implemented within the plane wave pseudopotential framework. It is found that the behavior of the electron density under external fields depends to a large extent on the geometrical structure of the molecule in question, rather than simply on the constituent functional groups. The normal modes are able to help distinguish between the different types of intramolecular hydrogen bonding present, and help to explain why leucine is found in the zwitterionic form for the gaseous phase. Calculated IR spectra show a marked difference between those obtained for zwitterionic and nonzwitterionic molecules. These differences can be attributed to the different chemical and hydrogen bonds present. Effective dynamical charges are calculated, and compared to atomic charges obtained from Mulliken population analysis. It is found that disagreement exists, largely due to the differing origins of these quantities.

Time dependent density functional theory study of charge-transfer and intramolecular electronic excitations in acetone–water systems

A recently introduced formulation of time dependent linear response density functional theory within the plane-wave pseudopotential framework [J. Hutter, J. Chem. Phys. 118, 3928 (2003)] is applied to the study of solvent shift and intensity enhancement effects of the A21 n→π* electronic transition in acetone, treating solute and solvent at the same level of theory. We propose a suitable formalism for computing transition intensities based on the modern theory of polarization, which is applicable to condensed-phase and finite systems alike. The gain in intensity brought about by thermal fluctuations is studied in molecular acetone at room temperature, and in gas-phase (CH3)2CO⋅(H2O)2 at 25 K. The latter system is characterized by the appearance of relatively intense features in the low-energy region of the spectrum, attributable to spurious solvent→solute charge-transfer excitations created by deficiencies in the DFT methodology. The n→π* transition can be partially isolated from the charge-transfer bands, yielding a blueshift of 0.17 eV with respect to gas-phase acetone. This analysis is then carried over to a solution of acetone in water, where further complications are encountered in the from of a solute→solvent charge transfer excitations overlapping with the n→π* band. The optically active occupied states are found to be largely localized on either solute or solvent, and using this feature we were again able to isolate the physical n→π* band and compute the solvatochromic shift. The result of 0.19 eV is in good agreement with experiment, as is the general increase in the mean oscillator strength of the transition. The unphysical charge transfers are interpreted in terms of degeneracies in the spectrum of orbital energies of the aqueous acetone solution.

Adhesive energy and charge transfer for MgO/Cu heterophase interfaces.

Local-density-functional-theory (LDFT) calculations within the plane-wave pseudopotential framework are performed for two polar (111) and two nonpolar (100) MgO/Cu interfaces. The polar interfaces have larger works of adhesion than nonpolar ones, consistent with field-ion and electron-microscopy observations of (111) interfaces in internally oxidized specimens. Large shifts in the potential of the interface layer relative to the bulk occur in the polar but not in the nonpolar interfaces. For both polar and nonpolar interfaces, the charge transfer profile is essentially confined to two layers on each side of the interface. \textcopyright{} 1996 The American Physical Society.