Constrained Density Functional Theory Calculations Research Articles

Creation and repair of luminescence defects in hexagonal boron nitride by irradiation and annealing for optical neutron detection

Hexagonal boron nitride (hBN) is promising for applications in neutron detection and quantum sensing. Here we created spin-dependent luminescence defects in hBN using 50 keV helium ion beam. The irradiated hBN flakes with a dose around 2 × 1014–1 × 1015/cm2 have the maximum photoluminescence (PL) intensity. The PL spectra are in the range of 700–1000 nm with a peak around 818 nm at 297 K after irradiation with a dose of less than 2 × 1014/cm2, which can be eliminated by high temperature annealing at 1097 K due to defects repair according to Raman spectra. However, the PL peaks exhibit a redshift from 818 nm to 830 nm when the irradiation dose is no less than 1 × 1015/cm2. Combining with constrained density functional theory calculations, it is found that high dose irradiation can cause local lattice swelling and further induce the redshift of PL spectra by 12 nm/0.33% strain. Furthermore, we proposal a novel optical neutron detection method by recording the intensity and position of PL signal of luminescence defects induced by neutron-10B nuclear reaction daughter particles. This work would be helpful to create luminescence defects in hBN for quantum sensing, and develop a novel neutron detector.

Constrained density functional theory calculations for estimation of forward and backward intermolecular charge transfer energy

AbstractCharge transfer (CT) in donor–acceptor complexes can occur in two directions: from donor to acceptor (forward CT, CTF) and from acceptor to donor (backward CT, CTB). For an ethylene and tetrafluoroethylene model system, coupled cluster calculations predict that CTF is more stable than CTB. Meanwhile, density functional theory (DFT) and time‐dependent DFT (TDDFT) employing B3LYP functional show a different CT state order depending on the Hartree–Fock exchange (HFX) contribution. The impact of basis sets as a possible cause to induce such unreasonable CT state reverse along with the increase of HFX was investigated. The physical origin of CT state reversal is examined in terms of molecular orbital energy. We analyzed two additional molecular pairs that feature CT state reversal along with the amount of HFX in B3LYP functional. Despite the reduced gap, we suggest constrained DFT using M06‐HF functional as an effective approach to estimate two CT state energies on equal footing.

17O Electron Nuclear Double Resonance Analysis of Compound I: Inverse Correlation between Oxygen Spin Population and Electron Donation.

Although the activation of inert C-H bonds by metal-oxo complexes has been widely studied, important questions remain, particularly regarding the role of oxygen spin population (i.e., unpaired electrons on the oxo ligand) in facilitating C-H bond cleavage. In order to shed light on this issue, we have utilized 17O electron nuclear double resonance spectroscopy to measure the oxygen spin populations of three compound I intermediates in heme enzymes with different reactivities toward C-H bonds: chloroperoxidase, cytochrome P450, and a selenolate (selenocysteinyl)-ligated cytochrome P450. The experimental data suggest an inverse correlation between oxygen spin population and electron donation from the axial ligand. We have explored the implications of this result using a Hückel-type molecular orbital model and constrained density functional theory calculations. These investigations have allowed us to examine the relationship between oxygen spin population, oxygen charge, electron donation from the axial ligand, and reactivity.

Pressure-dependent photoluminescence of Eu-activated aluminate hydride Sr3−x AxAlO4H:Eu2+ (A = Ca, Ba; x = 0, 1): Application of advanced U-determination technique for luminescence wavelength prediction

The increasing attention on the unique properties of oxyhydride materials motivates the exploration of their potential applications in optical fields, and the theoretical studies of their luminescence properties are still under progress. Here, we report the experimental and theoretical high-pressure photoluminescence (PL) studies on Eu-activated Sr3–xAxAlO4H (A = Ca and Ba; x = 0 and 1) oxyhydride materials. Under hydrostatic pressures from ambient pressure up to 6.41 GPa, the luminescence band in all the samples exhibits redshift with increasing pressure and the highest energy-shift rate of −101.85 cm−1/GPa was observed in Sr3AlO4H:Eu2+. The asymmetric bands were deconvoluted into two peaks corresponding to the two Eu sites with different coordination environments. Although the shift rates of Eu2+ centers in Sr3AlO4H are not remarkable as expected for the large compressibility of hydride ion ligands, their pressure-dependences in opposite directions were successfully reproduced by constrained density functional theory calculations using the advanced on-site Coulomb interaction parameter (U) determination method. The lower shift rate as seen in conventional oxide phosphors indicates that Eu-4f and 5d level positions are determined by the interaction with less compressive oxide ion ligands. Therefore, the high shift rate required for pressure sensing applications is expected in more hydrogen-rich oxyhydrides and related hydride compounds.

Implementation and Validation of Constrained Density Functional Theory Forces in the CP2K Package.

Constrained density functional theory (CDFT) is a powerful tool for the prediction of electron transfer parameters in condensed phase simulations at a reasonable computational cost. In this work we present an extension to CDFT in the popular mixed Gaussian/plane wave electronic structure package CP2K, implementing the additional force terms arising from a constraint based on Hirshfeld charge partitioning. This improves upon the existing Becke partitioning scheme, which is prone to give unphysical atomic charges. We verify this implementation for a variety of systems: electron transfer in (H2O)2+ in a vacuum, electron tunnelling between oxygen vacancy centers in solid MgO, and electron self-exchange in aqueous Ru2+–Ru3+. We find good agreement with previous plane-wave CDFT results for the same systems, but at a significantly lower computational cost, and we discuss the general reliability of condensed phase CDFT calculations.

Molecular Dynamics Simulations of Bimolecular Electron Transfer: the Distance-Dependent Electronic Coupling.

Understanding the distance dependence of the parameters underpinning Marcus theory is imperative when interpreting the results of experiments on electron transfer (ET). Unfortunately, most of these parameters are difficult or impossible to access directly with experiments, necessitating the use of computer simulations to model them. In this work, we use molecular dynamics simulations in conjunction with constrained density functional theory calculations to study the distance dependence of the electronic coupling matrix element, |HRP|, for bimolecular ET. Contrary to what is typically assumed for such intermolecular reactions, we find that the magnitude of |HRP| does not decay exponentially with the center-of-mass separation of the reactants, rCOM. The addition of other simple measures of donor/acceptor (D/A) orientation did not improve the correlation of |HRP| with rCOM. Using the minimum distance separation, rmin, of the reactants as the structural descriptor allowed the system to be partitioned into high-coupling/close-contact and low-coupling/non-contact regimes, but large fluctuations of |HRP| were still found for the close-contact reactant pairs. Despite the persistent large fluctuations of |HRP|, its mean value was found to decay piecewise exponentially with increasing rmin, which was attributed to significant changes in the average D/A pair structure. The results herein advise one to use caution when interpreting the experimental results derived from spherical reactant models of bimolecular ET.

PyCDFT: A Python package for constrained density functional theory.

We present PyCDFT, a Python package to compute diabatic states using constrained density functional theory (CDFT). PyCDFT provides an object-oriented, customizable implementation of CDFT, and allows for both single-point self-consistent-field calculations and geometry optimizations. PyCDFT is designed to interface with existing density functional theory (DFT) codes to perform CDFT calculations where constraint potentials are added to the Kohn-Sham Hamiltonian. Here, we demonstrate the use of PyCDFT by performing calculations with a massively parallel first-principles molecular dynamics code, Qbox, and we benchmark its accuracy by computing the electronic coupling between diabatic states for a set of organic molecules. We show that PyCDFT yields results in agreement with existing implementations and is a robust and flexible package for performing CDFT calculations. The program is available at https://dx.doi.org/10.5281/zenodo.3821097.

Nephelauxetic effect of the hydride ligand in Sr2LiSiO4H as a host material for rare-earth-activated phosphors.

Strontium lithium orthosilicate hydride Sr2LiSiO4H was synthesized by the reaction of Sr2SiO4 with LiH at 700 °C in a H2 rich atmosphere. Rietveld refinement of the neutron powder diffraction pattern revealed that Sr2LiSiO4H is isostructural to Sr2LiSiO4F (space group P21/m) and its channel-like structure preferentially accommodates H− ions over F− ions. In addition, Sr2LiSiO4H is stable in air and its Eu2+-doped analog exhibits yellow photoluminescence with an emission band at 544 nm and a broad excitation band ranging from 250 to 450 nm. These bands were observed in the longer wavelength region when compared with those displayed by Sr2LiSiO4F:Eu2+. The red shift, which is induced by H− substitution, is consistent with the constrained density functional theory calculations, predicting the photo-excitation and emission energies of 4f–5d transitions. The present study reports the synthesis of stable oxyhydrides acting as phosphor hosts for rare earth ions. The phosphor hosts exhibit large nephelauxetic effects owing to the presence of H− ligands.

Firefly Algorithm Applied to Noncollinear Magnetic Phase Materials Prediction.

In most noncollinear crystal magnets, the number of metastable states is quite large and any calculation that tries to predict the ground state can fall into one of the possible metastable phases. In this work, we generalize the population based meta-heuristic firefly algorithm to the problem of the noncollinear magnetic phase ground state prediction within density functional theory (DFT). We extend the different steps in the firefly algorithm to this specific problem by using polarized constrained DFT calculations, whereby using Lagrange multipliers the directions of the atom magnetic moments remain fixed. By locking the directions of the magnetic moments at each search iteration, the method allows one to explore the entire Born-Oppenheimer energy surface of existing and physically plausible noncollinear configurations present in a crystal. We demonstrate that the number of minima can be large, which restrains the use of exhaustive searches.

Effect of Binding Geometry on Charge Transfer in CdSe Nanocrystals Functionalized by N719 Dyes to Tune Energy Conversion Efficiency

Semiconductor quantum dots (QDs) functionalized by metal–organic dyes show great promise in photocatalytic and photovoltaic applications. However, the charge transfer direction and rates—key processes governing the efficiency of energy conversion—are strongly affected by the QD–dye interactions, insights on which are challenging to obtain experimentally. We use density functional theory (DFT) and constrained DFT calculations to investigate a degree of sensitivity of the electronic level alignment and related QD–dye electronic couplings to binding conformations of N719 dye at the surface of the 1.5 nm CdSe QD. Our calculations reveal a lack of direct correlations between the strength of the QD–dye interaction in terms of their binding conformations and the donor–acceptor electronic couplings. While the QD–dye binding conformations are the most stable when the N719 dye is attached to the QD via two carboxylate groups, the strongest electronic coupling between the QD as an electron donor and the dye as an el...

Adiabatic and Nonadiabatic Charge Transport in Li-S Batteries

The insulating nature of the redox end members in Li-S batteries, a-S and Li2S, has the potential to limit the capacity and efficiency of this emerging energy storage system. Nevertheless, the mechanisms responsible for ionic and electronic transport in these materials remain a matter of debate. The present study clarifies these mechanisms – in both the adiabatic and nonadiabatic charge transfer regimes – by employing a combination of hybrid-functional-based and constrained density functional theory calculations. Charge transfer in Li2S is predicted to be adiabatic, and thus is well described by conventional DFT methodologies. In sulfur, however, transitions between S8 rings are nonadiabatic. In this case, conventional DFT overestimates charge transfer rates by up to 2 orders of magnitude. Delocalized holes, and to a lesser extent, localized electron polarons, are predicted to be the most mobile electronic charge carriers in a-S; in Li2S hole polarons dominate. Although all carriers exhibit extremely low equilibrium concentrations, and thus yield negligible contributions to the conductivity, their mobilities are sufficient to enable the sulfur loading targets necessary for high energy densities. Our results highlight the value of methods capable of capturing nonadiabadicty, such as constrained DFT. These techniques are especially important for molecular crystals such as a-S, where longer-range charge transfer events are expected. Combining the present computational results with prior experimental studies, we conclude that low equilibrium carrier concentrations are responsible for sluggish charge transport in a-S and Li2S. Thus, a potential strategy for improving the performance of Li-S batteries is to increase the concentrations of holes in these redox end members.

Adiabatic and Nonadiabatic Charge Transport in Li–S Batteries

The insulating nature of the redox end members in Li–S batteries, α-S and Li2S, has the potential to limit the capacity and efficiency of this emerging energy storage system. Nevertheless, the mechanisms responsible for ionic and electronic transport in these materials remain a matter of debate. The present study clarifies these mechanisms—in both the adiabatic and nonadiabatic charge transfer regimes—by employing a combination of hybrid-functional-based and constrained density functional theory calculations. Charge transfer in Li2S is predicted to be adiabatic, and thus is well-described by conventional DFT methodologies. In sulfur, however, transitions between S8 rings are nonadiabatic. In this case, conventional DFT overestimates charge transfer rates by up to 2 orders of magnitude. Delocalized holes, and to a lesser extent, localized electron polarons, are predicted to be the most mobile electronic charge carriers in α-S; in Li2S, hole polarons dominate. Although all carriers exhibit extremely low equ...

Hydrogen Treatment as a Detergent of Electronic Trap States in Lead Chalcogenide Nanoparticles

Lead chalcogenide (PbX) nanoparticles are promising materials for solar energy conversion. However, the presence of trap states in their electronic gap limits their usability, and developing a universal strategy to remove trap states is a persistent challenge. Using calculations based on density functional theory, we show that hydrogen acts as an amphoteric impurity on PbX nanoparticle surfaces; hydrogen atoms may passivate defects arising from ligand imbalance or off-stoichiometric surface terminations irrespective of whether they originate from cation or anion excess. In addition, we show, using constrained density functional theory calculations, that hydrogen treatment of defective nanoparticles is also beneficial for charge transport in films. We also find that hydrogen adsorption on stoichiometric nanoparticles leads to electronic doping, preferentially n-type. Our findings suggest that postsynthesis hydrogen treatment of lead chalcogenide nanoparticle films is a viable approach to reduce electronic trap states or to dope well-passivated films.

Energetics of Photoinduced Charge Migration within the Tryptophan Tetrad of an Animal (6-4) Photolyase.

Cryptochromes and photolyases are flavoproteins that undergo cascades of electron/hole transfers after excitation of the flavin cofactor. It was recently discovered that animal (6-4) photolyases, as well as animal cryptochromes, feature a chain of four tryptophan residues, while other members of the family contain merely a tryptophan triad. Transient absorption spectroscopy measurements on Xenopus laevis (6-4) photolyase have shown that the fourth residue is effectively involved in photoreduction but at the same time could not unequivocally ascertain the final redox state of this residue. In this article, polarizable molecular dynamics simulations and constrained density functional theory calculations are carried out to reveal the energetics of charge migration along the tryptophan tetrad. Migration toward the fourth tryptophan is found to be thermodynamically favorable. Electron transfer mechanisms are sought either through an incoherent hopping mechanism or through a multiple sites tunneling process. The Jortner-Bixon formulation of electron transfer (ET) theory is employed to characterize the hopping mechanism. The interplay between electron transfer and relaxation of protein and solvent is analyzed in detail. Our simulations confirm that ET in (6-4) photolyase proceeds out of equilibrium. Multiple site tunneling is modeled with the recently proposed flickering resonance mechanism. Given the position of energy levels and the distribution of electronic coupling values, tunneling over three tryptophan residues may become competitive in some cases, although a hopping mechanism is likely to be the dominant channel. For both reactive channels, computed rates are very sensitive to the starting protein configuration, suggesting that both can take place and eventually be mixed, depending on the state of the system when photoexcitation takes place.

Fragment approach to constrained density functional theory calculations using Daubechies wavelets.

In a recent paper, we presented a linear scaling Kohn-Sham density functional theory (DFT) code based on Daubechies wavelets, where a minimal set of localized support functions are optimized in situ and therefore adapted to the chemical properties of the molecular system. Thanks to the systematically controllable accuracy of the underlying basis set, this approach is able to provide an optimal contracted basis for a given system: accuracies for ground state energies and atomic forces are of the same quality as an uncontracted, cubic scaling approach. This basis set offers, by construction, a natural subset where the density matrix of the system can be projected. In this paper, we demonstrate the flexibility of this minimal basis formalism in providing a basis set that can be reused as-is, i.e., without reoptimization, for charge-constrained DFT calculations within a fragment approach. Support functions, represented in the underlying wavelet grid, of the template fragments are roto-translated with high numerical precision to the required positions and used as projectors for the charge weight function. We demonstrate the interest of this approach to express highly precise and efficient calculations for preparing diabatic states and for the computational setup of systems in complex environments.

Density-functional calculations of field-dependent ionization potentials and excitation energies of aromatic molecules

A recent method based on constrained density functional theory (CDFT) has been used to calculate the field-dependent ionization potential by determining the dissociation barrier for the interaction between a cation and an electron in an electric field. In the CDFT model, we rely on that the barrier is located somewhere outside the cation, which has limited the applicability for polyaromatic molecules where the barrier is located closer to the cation than for other molecules. Different density functionals, basis sets and choices of constraints in the CDFT calculations are tested for benzene as a case study. The field-dependent ionization potential calculated by constraining the charge with the B3LYP functional and the cc-pVDZ basis set shows a good agreement with our previous work and has a low computational cost for the larger aromatic molecules included here. In addition, field-dependent excitation energies are investigated using time-dependent DFT.

Resonant hot charge-transfer excitations in fullerene-porphyrin complexes: Many-body Bethe-Salpeter study

We study the neutral singlet excitations of the zinc-tetraphenylporphyrin and ${C}_{70}$-fullerene donor-acceptor complex within the many-body Green's-function $GW$ and Bethe-Salpeter approaches. The lowest transition is a charge-transfer excitation between the donor and the acceptor with an energy in excellent agreement with recent constrained density functional theory calculations. Beyond the lowest charge-transfer state, which can be determined with simple electrostatic models that we validate, the Bethe-Salpeter approach provides the full excitation spectrum. We evidence the existence of hot electron-hole states which are resonant in energy with the lowest donor intramolecular excitation and show a hybrid intramolecular and charge-transfer character, favoring the transition towards charge separation. Such findings, and the ability to describe accurately both low-lying and excited charge-transfer states, are important steps in the process of discriminating ``cold'' versus ``hot'' exciton dissociation processes.

Implementation of the DFT+U method and constrained DFT calculations for U and J within a pseudopotential formalism: Application to FeO and LaVO3

A first-principles density functional theory (DFT) + U method based on the pseudopotential (PP) method was developed to enable a-priori prediction of electronic structures of strongly-correlated electronic structure (SCES) systems. The method determines the radius of the localized electron orbitals of a SCES system and the Hubbard parameters U and J for the electrons and calculates the electronic structure of the system by using the rotationally-invariant DFT + U method and using the determined radius and parameters. The electronic structure of FeO in the anti-ferromagnetic state was calculated by using the method, and the method could describe the electronic structure of FeO correctly, producing an insulating ground state for FeO with an indirect band gap of 1.88 eV.

COMPUTATIONAL STUDY OF BRIDGE-MEDIATED INTERVALENCE ELECTRON TRANSFER II: COUPLINGS IN DIFFERENT METALLOCENE COMPLEXES

The constrained density functional theory (CDFT) was used to study bridge-mediated electron transfer processes in mixed-valence systems with two identical metallocene (cobaltocene, ruthenocene, and nickelocene) moieties linked by various bridge structures. Based on the electronic coupling matrix elements obtained from the CDFT calculations, the relationship between the bridge linkage and the effectiveness of intervalence transfer was discussed.

Quantum effects in biological electron transfer

Over recent decades, quantum effects such as coherent electronic energy transfers, electron and hydrogen tunneling have been uncovered in biological processes. In this Perspective, we highlight some of the main conceptual and methodological tools employed in the field to investigate electron tunneling in proteins, with a particular emphasis on the methodologies we are currently developing. In particular, we describe our recent contributions to the development of a mixed quantum-classical framework aimed at describing physical systems lying at the border between the quantum and semi-classical worlds. We present original results obtained by combining our approach with constrained Density Functional Theory calculations. Moving to coarser levels of description, we summarize our latest findings on electron transfer between two redox proteins, thereby showing the stabilization of inter-protein, water-mediated, electron-transfer pathways.