Hybrid Density Functional Study Research Articles

On the magnetic and thermodynamic properties of Americium-II: A hybrid density functional theoretic study

The ground states of the actinides have been matters of considerable controversies, theory often contradicting experiment specifically in regards to the magnetic nature. To resolve this discrepancy, we present here hybrid density functional theory (HYB-DFT) based studies of the structural, magnetic, electronic, and thermodynamic properties of Americium-II. Three configurations of non-magnetic (NM), anti-ferromagnetic (AFM), and ferromagnetic (FM) with and without spin–orbit coupling (SOC) have been considered to determine the ground state of Am-II. We find that the experimental NM ground state configuration is indeed obtained for Am-II at a level of 40% Hartree–Fock (HF) exchange with SOC and the computed structural properties and the electronic density of states are in good agreement with experimental observations. We also find that HBY-DFT with NSOC fails to predict the correct magnetic and electronic structures for Am-II, indicating the importance of the inclusion of SOC for studies of strongly correlated materials. The phonon related thermodynamic properties of Am-II are presented for the NM ground state configuration and the computed heat capacity and entropy are found to be in good agreement with the experimental measurements. The lattice constant, bulk modulus, heat capacity, and entropy of AM-II are predicted to be 9.44 a.u., 21.7 GPa, 24.3 J K − 1 mol − 1 , and 55.7 J K − 1 mol − 1 , respectively.

Electronic structure of oxygen-terminated zigzag graphene nanoribbons: A hybrid density functional theory study

The size-dependent electronic structure of oxygen-terminated zigzag graphene nanoribbons is investigated using standard density functional theory (DFT) with an exchange-correlation functional of the generalized gradient approximation form as well as hybrid DFT calculations with two different exchange-correlation functionals. Hybrid DFT calculations, which typically provide more accurate band gaps than standard DFT, are found to predict semiconducting behavior in oxygen-terminated zigzag graphene nanoribbons; this is in distinct contrast to standard DFT with (semi)local exchange-correlation functionals, which have been widely employed in previous studies and shown to predict metallic behavior. (Semi)local exchange-correlation functionals employed in standard DFT calculations cause unphysical delocalization of lone pairs from the oxygen atoms due to self-interaction errors and lead to metallic behavior. Hybrid DFT calculations do not suffer from this spurious effect and produce a clear size-dependent band gap. Appreciable fundamental band gaps $(\ensuremath{\sim}1\text{ }\text{eV})$ are found for the smallest ribbons (two zigzag rows); the band gap decreases rapidly with increasing ribbon width, resulting eventually in a zero band-gap semiconductor at about 4--5 zigzag rows. This finding could have useful implications for molecular electronics, in particular, since oxygen-terminated zigzag graphene nanoribbons are thermodynamically stable unlike their hydrogenated counterparts. More generally, through a concrete example, this study suggests caution when employing (semi)local functionals in DFT studies of functionalized graphene/graphene derivatives when the functional groups contain electron lone pairs.

Electronic Structure of (Ga1−xZnx)N1−xOx Photocatalyst for Water Splitting by Hybrid Hartree-Fock Density Functional Theory Methods

GaN:ZnO solid solutions have been identified as a promising system for photocatalytic or photoelectrochemical water splitting under visible-light irradiation. However, the origin of their activity at longer wavelength with respect to the parent materials (GaN and ZnO) absorbing in the UV spectrum is still matter of debate. Previous theoretical studies were based on standard GGA or GGA+U calculations which largely underestimated the band gap values of the two semiconductors and are thus not best suited methods for the required analysis. The present is a hybrid density functional study (B3LYP) which provides more accurate description of the electronic structure of the parent semiconductors and is thus also more reliable for the evaluation of the mixed GaN:ZnO system. For small concentrations of ZnO in GaN, local inhomogeneity of Zn or O concentration must be invoked to observe a red-shift of the absorption edge. For larger concentrations, some random alloy distributions, enthalpically more expensive but ent...

A screened hybrid density functional study on energetic complexes: Cobalt, nickel and copper carbohydrazide perchlorates

The molecular geometry, electronic structure, infrared spectra and thermochemical properties of cobalt and nickel tris(carbohydrazide) perchlorates (CoCP and NiCP) as well as copper bis(carbohydrazide) perchlorate (CuCP) were investigated using the Heyd–Scuseria–Ernzerhof (HSE) screened hybrid density functional. The results show that both perchlorate ions coordinate with the copper atom, and the interactions between copper and perchlorate are ionic, whereas all the metal–carbohydrazide interactions are covalent. Due to the delocalization from the σ N–H bond orbital to the n* M antibond orbital, the amino stretching vibrations of these complexes show considerable red-shift compared with those of free carbohydrazide ligand. The calculated heats of reaction and formation indicate that the formations of these complexes are exothermic, and the order of their thermal stability is NiCP > CoCP > CuCP. These agree well with the experimental results. Finally, we find that there is a relationship between the energy gap and impact sensitivity.

A hybrid density functional study of lithium in ZnO: Stability, ionization levels, and diffusion

The properties of interstitial and substitutional Li in wurtzite ZnO are modeled using hybrid density functional calculations. We investigate the impact of the band-gap error on the formation energies of the two defects and their dependence on the Fermi level. It is found that within a local-density approximation, the acceptor level of ${\text{Li}}_{\text{Zn}}$ is very close to the valence-band top but as the band gap is opened, the acceptor state becomes more localized and the respective level is shifted upward. Taking polaronic effects into account, we place the ionization level of ${\text{Li}}_{\text{Zn}}$ between ${E}_{v}+0.60\text{ }\text{eV}$ and ${E}_{v}+1.1\text{ }\text{eV}$. This deeper level explains the difficulty in realizing $p$-type ZnO using Li as monodopant. Further, the mobility of the defects was investigated. While interstitial Li is mobile at low temperatures, independent of the stoichiometry, the diffusion of ${\text{Li}}_{\text{Zn}}$ depends on the concentrations of intrinsic defects. Our calculations show that in O-rich material, where the defect is more stable, the dominant diffusion process corresponds to a dissociative mechanism requiring a substantial activation energy.

A screened hybrid density functional study on energetic complexes: Alkaline-earth metal carbohydrazide perchlorates

The molecular geometries, electronic structures and stabilities of a series of alkaline-earth metal carbohydrazide perchlorates were investigated using the Heyd–Scuseria–Ernzerhof (HSE) screened hybrid density functional. The results show that Be and Mg complexes have six-coordinated octahedron features, as previously reported for the transition metal complexes. However, Ca, Sr and Ba complexes have additional coordinated oxygen atoms from the perchlorate ion. Detailed NBO analyses indicate that the metal–ligand interactions are essentially ionic and play an important role in the stabilities of these energetic complexes. The donor–acceptor interactions result in a reduction of occupancies of σ C=O and σ N–H bond orbitals, and also their subsequent impact on bond length and bond order.

The structural and electronic properties of chiral SiC nanotubes: a hybrid densityfunctional study

Hybrid density functional theory was employed in investigating the structuraland electronic properties of 14 chiral and 3 armchair SiC nanotubes(SiCNTs). The role of the tube diameter, as well as that of the chiral angleϑ,was studied in detail by considering nanotubes of diameters varying from 2 to 9 Å and chiral anglesϑ varyingbetween 7° and 30°. The study revealed that all the investigated SiCNTs are semiconductors with abroad spectrum of bandgap values ranging from 0.2 to 2.9 eV and that thestructural stability of the nanotubes increases with diameter. By analyzing thebehavior of the molecular orbitals, an explanation of the mechanism by whichϑ affects the determination of such values is put forward.

Cr/Sb co-doped TiO2 from first principles calculations

Co-doping of TiO2 with Cr and Sb was recently found to have a beneficial effect on the photocatalytic activity under visible-light irradiation. With the present comparative standard and hybrid density functional study we get new insight into the electronic structure of the system and provide a theoretical support to the experimental findings. Hybrid DFT better describes (i) the Cr d states splitting and thus the semiconducting properties of the Cr-doped system, and (ii) the localized nature of the Ti3+ states produced by Sb doping. An electron transfer from the Sb-induced states to the Cr 3d levels is considered to be the reason for the enhanced photostability of the co-doped Cr/Sb system.

On the existence and stability of single walled SiGe nanotubes

A systematic hybrid density functional theoretical study of three different types of single walled SiGe nanotubes in armchair ( n, n) and zigzag ( n, 0) (3 ⩽ n ⩽ 11) configurations is presented. The nanotubes have been represented by finite clusters with the dangling bonds saturated by hydrogen atoms. All structures have been both geometry and spin optimized without any symmetry constraints. Stability trend differs among armchair and zigzag nanotubes for the three different types. However, all SiGe nanotubes are semiconducting in nature, with a wide spectrum of band gaps.

Electronic structures of defects in ZnO: Hybrid density functional studies

Theoretical investigation of the defect energy levels in band gap for ZnO is of great importance to understand the photoluminescence (PL) spectrum in experiment. However, the "band-gap" problem in the density functional calculations leads to incorrect evaluation of the defect levels inside the band gap. In this work, we perform hybrid density functional calculation, which can accurately predict the band gap width and the gap states, to study the electronic structures of both native defects and doped impurities of Li, Na, K, and Ag in ZnO. Our results show that both the vacancies and octahedral self-interstitials of O and Zn contribute to the broad green PL spectrum observed in experiments. In addition, the gap states arising from these doped impurities are predicted, where some of the gap states match with the experimental values. Moreover, the characters of these gap states are discussed based on the calculated density of states.

A solid-state hybrid density functional theory study of Prussian blue analogues and relatedchlorides at pressure

The variations with pressure in the structural, electronic andmagnetic properties of a series of Prussian blue analogues (PBAs)KIMII[CrIII(CN)6] (M = VII,MnII andNiII) and associatedisomorphous chlorides KIMIICrIIICl6 are investigated within a series of solid-state hybrid density functional calculations. The sensitivityof the computed properties to the choice of Hamiltonian is tested by application of functionalscontaining 35%, 65% and 100% admixtures of Fock exchange. Magnetic coupling constants(J) are obtained at arange of cell volumes (V), with fits of the Bloch relationship (, ε typically 3–4)yielding exponents ε in the ranges 5.16–6.34, 8.48–12.07 and 4.00–4.51 for the antiferromagnetic (AF)VIICrIII-, ferrimagnetic(FI) MnIICrIII- andferromagnetic (FO) NiIICrIII PBAs, respectively; and 3.33–4.99, 1.86–3.09 and 1.65–3.28 for the AFVIICrIII-, FOMnIICrIII- and FONiIICrIII chlorides, respectively.The MnIICrIII PBA rangeencloses the high values ε∼9–10 obtained in a recent joint experimental and theoretical study, and it issuggested that this strong magnetostructural effect arises due to the presenceof competing AF and FO interactions in this material. Estimates of the spinordering temperatures derived from the combination of the 35%-functional couplingswith a mean field approach are in good agreement with experiment in theVIICrIII andNiIICrIII PBAs, but aretoo low in the MnIICrIII system. The variations with pressure in the structural parameters, charges and spinmoments are also detailed, the PBA and chloride energy–volume data yielding bulk moduliin the ranges 39–53 and 36–50 GPa, respectively. Finally, the energies governingCN– ligand isomerization are estimated and successfully interpreted in terms of a simple crystalfield stabilization model.

On the electronic and geometric structures of armchair GeC nanotubes: a hybrid densityfunctional study

Ab initio calculations within the framework of hybrid density functional theoryand the finite cluster approximation have been performed for the electronic andgeometric structures of three different types of armchair germanium carbide nanotube,from (3, 3) to (11, 11). Full geometry and spin optimizations with unrestrictedsymmetry have been performed. Physically pertinent quantities of interest suchas the cohesive energies, band gaps, radial buckling, density of states, dipolemoments, and Mulliken charge distributions have been investigated in detail forall nanotubes. For type I nanotubes, the largest cohesive energy obtained is4.092 eV/atom, whereas for type II and type III nanotubes, the values are3.987 eV/atom and 3.968 eV/atom, respectively. For optimized type I nanotubes, Ge atoms moved toward the tube axis andC atoms moved in the opposite direction after relaxation, opposite to the trends observedin types II and III. The band gaps for type I nanotubes are larger than the bulk 3C-GeCgap, varying between 2.666 and 3.016 eV, while type II and type III nanotubes havesignificantly lower band gaps, with all nanotubes being semiconducting in nature. Mullikencharge analysis indicates primarily ionic behavior for type I GeC nanotubes and amixed ionic with covalent behavior for the other two types. None of the tubesappear to be magnetic. Applications in the field of nano-optoelectronic devices,molecular electronics, and band gap engineering are envisioned for GeC nanotubes.

The screened hybrid density functional study of metallic thorium carbide

The B1 rock-salt-type phase of thorium carbide is studied using Gaussian-type-orbitals and density functional theory using periodic boundary conditions. The functionals used range from pure (LSDA), generalized gradient approximation (PBE), and PBEsol for solids, as well as screened hybrid (HSE) functional. Calculated structural parameters, bulk moduli, density of states, and band structures are presented and compared with previously reported plane-wave studies. Unlike previous studies of actinide oxides, HSE does not open a band gap near the Fermi energy in ThC, thus agreeing with the metallic character predicted by LSDA and PBE.

A Hybrid Density Functional Study of Zigzag and Chiral Si Nanotubes

A Hybrid Density Functional Study of Zigzag and Chiral Si Nanotubes

Hybrid density functional study of armchair SiC nanotubes

First-principles calculations for the electronic and geometric structures of three different types of armchair silicon carbide nanotubes from (3, 3) to (11, 11) have been performed using hybrid density functional theory and the finite cluster approximation. Full geometry and spin optimizations have been performed without any symmetry constraints. A detailed comparison of the structures and stabilities of the three types of nanotubes is presented. For type 1 nanotube, the cohesive energy appears to saturate at $4.63\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, whereas for type 2 and 3 nanotubes, the cohesive energy saturates at approximately $4.44\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The dependence of the electronic band gaps on the respective tube diameters, energy density of states, and dipole moments as well as Mulliken charge distributions have been investigated. For type 1 nanotubes, Si atoms moved toward the tube axis and C atoms moved in the opposite direction after relaxation, consistent with other SiC nanotubes found in literature. For type 2 and the newly proposed type 3, this displacement direction is reversed. The band gaps for type 1 nanotubes are larger than bulk $3C\text{\ensuremath{-}}\mathrm{Si}\mathrm{C}$ gap, varying between 2.78 and $2.91\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, while type 2 and type 3 nanotubes have significantly lower band gaps. Unlike the other two types, band gap for type 3 nanotubes decreases monotonically with increasing tube diameter from $1.22\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ for the smallest tube to $0.79\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ for the largest (11, 11) tube studied here. The corresponding numbers for type 2 are 1.49 and $0.91\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ with an oscillatory pattern. None of the tubes appear to be magnetic. It is expected that these tubes will have interesting and important applications in the field of band gap engineering and molecular electronics.

CO adsorption on metal surfaces: a hybrid density functional study with plane wave basisset (abstract only)

We present a detailed density-functional investigation of the adsorption of CO in top andhollow sites on the (111) surfaces of Cu, Rh and Pt. The study has been performed using aconventional gradient-corrected exchange–correlation functional (the PBE functionalproposed by Perdew, Burke and Ernzerhof 1996 Phys. Rev. Lett. 77 3865) and two hybridfunctionals mixing density-functional and exact (Hartree–Fock) exchange (the PBE0functional of Perdew, Burke and Ernzerhof 1996 J. Chem. Phys. 105 9982 and the HSE03functional proposed by Heyd, Scuseria and Ernzerhof 2003 J. Chem. Phys.118 8207).As expected, the gradient corrected functional shows the tendency to favor the hollow sites,at variance with experimental findings that give the top site as the most stable adsorptionsite.The PBE0 and HSE03 functionals reduce this tendency. Indeed, they predict the correctadsorption site for Cu and Rh but fail for Pt. But even in this case, the hybrid functionaldestabilizes the hollow site by 50 meV compared to the generalized gradient approximationdescription. This suggests that, in general, hybrid functionals still do not predict thecorrect adsorption site.

Hybrid density functional study of the oxidized states of NiFe-hydrogenase

Abstract The oxidized states of NiFe-hydrogenase and their activations have been studied using hybrid DFT. For Ni-B, which is activated in seconds, there is good agreement with experiments. The structure has a bridging hydroxide, which can be removed in two ways. In one pathway an outside electron and a proton is added and water removed, while the other pathway does not involve any outside electrons but uses a hydrogen molecule instead. For Ni-A, which can take hours to activate, there appears to be a rather severe discrepancy with experiments. Even though a structure was obtained in good agreement with experiment, with a protonated peroxide side-on bonded to nickel, this is not the lowest-energy structure. Instead, a structure with a bridging, end-on, protonated peroxide is found to be much lower in energy even for the largest model used with nearly 120 atoms. A remaining deficiency in the chemical model is the most likely explanation for the error. An error of the hybrid DFT method is also possible but appears much less likely, since the agreement between hybrid DFT and non-hybrid DFT is very good. The side-on structure of Ni-A is activated by first adding an outside electron and a proton and then cleaving the O–O bond. Altogether, three outside electrons are required to remove the peroxide, which could be a reason for the slow activation of Ni-A.

Hybrid density functional study of ligand coordination effects on the magnetic couplings and the dioxygen binding of the models of hemocyanin

AbstractLigand coordination effects on the magnetic interaction and the reversible dioxygen binding process for the models of hemocyanin have been investigated with hybrid density functional theory. Hemocyanin is an oxygen transport protein found in mollusks and arthropods. The active site of hemocyanin contains a pair of copper atoms, each of which is coordinated by three histidine residues. We utilized NH3 (a), methylimidazole (b), and histidine (c) as the ligands of the models of hemocyanin, which coordinate to the binuclear copper core in the active site of oxygenated (1z) (z = a–c) and deoxygenated hemocyanin (2z). The difference of the ligand coordination in the magnetic couplings for the model of hemocyanin 1z complexed with peroxide (1z‐O) shows that the substitution from histidine to methylimidazole group is not serious, but that the substitution from imidazole to NH3 is influential. In the dioxygen binding process, the ligands influence the binding energy and the shapes of potential energy surfaces of the models of hemocyanin 1z and 2z with dioxygen due to the strength of the mixing between the ground state and the charge transfer one. The active dxy ± dxy orbitals of the copper core of 1b and 1c strongly interact with π and π orbitals of the dioxygen than those of 1a, leading to the larger binding energy of the model 1b (1c) with dioxygen. The orbital energies of the active orbitals for the synthetic model, [Cu(HB(3,5‐iPr2‐Pz)3)]2 (HB(3,5‐iPr2‐Pz)3 = hydrotris{3,5‐diisopropyl‐pyrazolyl}borate), imply that it irreversibly binds dioxygen because HB(3,5‐iPr2‐Pz)3 groups show stronger electron‐release character and raise the orbital energies nearer that to π* orbitals of O2. The effective bond‐order, localized electron density, and information entropy for 1z with dioxygen were calculated in terms of occupation numbers of the natural orbitals of the broken‐symmetry UB2LYP solution in order to elucidate the nature of the Cu2O2 bonding. These chemical indices indicate that the copper–oxygen bonds in Hc exhibit an intermediate covalent‐bonding character (half bonding), which is the origin of an active control of oxygen transport: addition and dissociation of O2 in hemocyanin by the conformational changes. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007

The B3LYP hybrid density functional study on solids

The B3LYP hybrid density functional method, which is very successful in the study of thermochemistry of atoms and molecules, has been applied to some periodic systems recently. The applications to solids and surfaces show that the B3LYP hybrid functional reproduces the experimental energy gaps and magnetic moments for a variety of materials.

Hybrid density functional theory study of fragment ions generated during mass spectrometry of 1,3‐dioxane derivatives

It was recently reported that the cis,cis and trans,trans diastereoisomers of four 2(r)-R-2,4(R),6(S)-trimethyl-1,3-dioxane derivatives show distinct electron ionization mass spectra. As a possible explanation for this finding, the authors suggested that the ions generated during the mass spectrometry of these compounds could follow different fragmentation patterns that initiate from different ion conformations. In this report, hybrid density functional theory methods have been used to investigate the conformational preference of three ions involved in the mass spectrometry of some 1,3-dioxane derivatives. We found that there is indeed more than one stable ion conformation for each of the investigated ions. Energy profiles along the torsional coordinates connecting the conformers are presented, and factors influencing the relative stability of ion conformations are discussed.