Discrete Variational Xα Research Articles

Exploring the Luminescent Properties of Transition Metal Ions in Solids through Theoretical Insights and the Prospects of Machine Learning

This work delineates over a decade of rigorous first-principles computational investigations into compounds doped with transition metal (TM) ions, such as Sc3+, Ti3+, V2+, V3+, Cr3+, Mn3+, Mn4+, Fe3+, Co3+, Ni3+, and Cu3+, within host matrices including α-Al2O3, MgTiO3, and various fluorides, devoid of empirical parameters. Our methodological framework employs discrete variational Xα (DV-Xα) and discrete variational multielectron (DVME) techniques, enhanced by configuration-dependent and correlation corrections to refine accuracy. Notably, our analysis integrates lattice-relaxation estimations through the use of Shannon crystal radii and the Cambridge Serial Total Energy Package (CASTEP), facilitating the examination of pressure-induced effects on lattice structures.Our comprehensive studies, particularly on α-Al2O3 doped with TM 3d3 ions, have not only successfully replicated known optical characteristics but have also provided new insights into the variation of transition energies across different TM ion charges, alongside pressure-dependent behavior of multiplet energies in alignment with experimental findings. These endeavors highlight the complex interplay of dopant characteristics, including charge states and ionic radii, on the optical properties of host crystals, with precise attention to colorimetry within the International Commission on Illumination (abbreviated CIE) color space framework to evaluate the quality and perception of emitted light.Furthermore, this work will introduce a discussion on the emerging potential of machine learning (ML) in advancing our understanding and predictive capabilities concerning the luminescence of materials. ML approaches promise to revolutionize the identification of optimal dopant-host combinations, predict novel luminescent materials, and enhance the accuracy of theoretical predictions through data-driven insights. By bridging traditional computational methods with ML, we aim to pave new pathways in the exploration of luminescent materials, potentially leading to innovative applications in lighting, display technologies, and beyond.

First-Principles Analysis of the Influence of the Valence on the Optical Spectra of Uranium Ions in Oxide Crystals

Recently U6+-doped oxide green phosphors are attracting attentions for the white LED application. However, the absorption and emission mechanisms in some of these materials are still unclear. For the development of the novel uranium-doped phosphors, clarification of the origins of the peaks in the absorption and emission spectra of these materials is indispensable.In our previous research, we investigated the origin of the peaks in the optical spectra of uranium ions in some oxide crystals by comparing the experimental spectra with the theoretical ones obtained by first-principles calculations. The results indicated that there is a U-doped phosphor whose spectrum seems to originate from 5f-6d transitions of U3+ instead of LMCT transitions of U6+. In this work, in order to investigate the influence of the valence of uranium ions, we created some hypothetical model clusters representing uranium ions in oxides and performed first-principles calculations for the LMCT transitions of U6+ and the 5f-6d transitions of U3+ using the relativistic discrete-variational multi-electron (DVME) code which is a configuration-interaction calculation program based on the four-component fully relativistic wave functions obtained by the relativistic discrete-variational Xα (DV-Xα) molecular orbital method. The effects of coordination numbers and bond distances on valence of uranium ions were also investigated in detail by performing calculations using model clusters with coordination numbers from 6 to 12.

Optimization of First-Principles Calculation Conditions for Ce3+ in Perovskite-Type Oxides

The 4f-5d transition of Ce3+ in crystals is used in a variety of optical materials, including scintillators and phosphors. It also has the advantages that the wavelength can be controlled by the coordination environment and that Ce3+ is relatively inexpensive among rare earth elements. For the theoretical design of novel Ce3+-doped phosphors, prediction of 4f-5d transition energy by first-principles molecular orbital calculations using model clusters is effective. In general, increase of the cluster size improves the accuracy of calculations and reduces the difference between the experimental and calculated values while very large clusters have the disadvantage of requiring a huge amount of calculation time. Hence, understanding the cluster size dependence is important to determine the efficient and effective cluster size for the first-principles calculations.In this study we focused on Ce3+-doped in YAlO3, and performed first-principles calculations of the 4f-5d transition energies for model clusters with different sizes. At first, the local structures around the Y3+ site of YAlO3 were cut out from the crystal structure. Then the central Y3+ ions of these clusters were replaced by Ce3+ ions. Then the transition energy of Ce3+ in YAlO3 was calculated by the relativistic discrete-variational Xα (DV-Xα) molecular orbital calculations using the Slater's transition state method. The detailed comparison of the results showed that for clusters larger than a certain size, the error between the experimental and calculated values became negligibly small. For comparison, the lattice relaxation was also taken into account by modifying the model clusters using two different methods: the method based on the Shannon's crystal radii and the structural optimization using the CASTEP code, and the results were compared in detail.

First-Principles Calculations of the Optical Spectra of Uranium Ions in Oxide Crystals

Recently U6+-doped phosphors are attracting attentions as potential green phosphors for the white LED application. However, the origins of the peaks in the optical spectra in some U-doped phosphors have not been clarified yet. For the development of the novel uranium phosphors, detailed analysis of the absorption and emission mechanisms of these materials is indispensable.In this study, we have performed the first-principles electronic structure calculations of U in oxide crystals using the relativistic discrete-variational multi-electron (DVME) code which is a configuration-interaction calculation program based on the four-component fully relativistic wave functions obtained by the relativistic discrete-variational Xα (DV-Xα) molecular orbital method. The optical spectra due to the charge transfer transitions of U6+ in oxides were calculated using the explicitly obtained many-electron wave functions. For comparison, the 5f-6d transitions of U3+ in oxide crystals were also calculated and the influence of the valency was investigated. The theoretical spectra were compared with the experimental spectra and the origins of the peaks were clarified based on the configuration analysis of the corresponding relativistic many-electron wave functions.

Theoretical Analysis of Anomalous Emission of Eu2+ in Crystals By First-Principles Calculation

The 5d-4f transition energies of lanthanides are very versatile, especially the 4f65d1-4f75d0 transition energy of Eu2+ has high luminescence efficiency and is used in various luminescent materials such as phosphors like SrxCa1-xAlSiN3: Eu2+, scintillators, and solid-state lasers, etc. However, there are materials that do not undergo pure 5d-4f transitions, showing the so-called anomalous luminescence instead. The anomalous luminescence is caused by the formation of excitons due to the overlap of the 5d level of Eu2+ with the conduction band and the transition of electrons from the conduction band to the 4f level, resulting in a shift to a lower energy than the general 5d-4f transition energy. A more detailed analysis of this phenomenon is important for the development of novel luminescent materials using Eu2+.In this study, we aimed to theoretically analyze the anomalous luminescence based on first-principles electronic structure calculations. The model clusters of Eu2+-doped crystals consisting of Eu2+ at the substitution site and the surrounding ions were created. Then molecular orbital calculations were performed by the relativistic discrete-variational Xα (DV-Xα) method and subsequently multiplet calculations were performed by the relativistic discrete-variational multi-electron (DVME) method. These first-principles calculations were carried out for Eu2+-doped crystals that show anomalous luminescence and those that do not show anomalous luminescence, and the electronic states of these materials were compared in detail.

Prediction of 4f-5d Transition Energy of Ce3+ in Oxides By First-Principles Calculations and Machine Learning

A Ce3+ ion has only one electron in the 4f orbitals and its 4f-5d transition energy is strongly influenced by the width of the crystal field splitting of the 5d orbitals. In other words, the absorption and emission wavelengths of Ce3+ in crystals can be widely controlled by changing the ligand field environment. As an example of Ce3+-doped phosphor, Ce:YAG (Y3Al5O12) is widely used as an yellow phosphor for standard white LEDs. Therefore, novel phosphors with desired emission colors can be developed by adding Ce3+ to appropriate host crystals.In this work, we aimed to construct an efficient prediction model for the transition energy of Ce3+ in oxide crystals. Small clusters consisting of the substituted Ce3+ ion and the first-neighbor anions were constructed and molecular orbital calculations were performed using the relativistic discrete-variational Xα (DV-Xα) method. Then machine learning based on structural and electronic state parameters was performed to create a predictive model for the 4f-5d transition energy of Ce3+ in crystals. By creating the correlation diagrams, the influence of each parameter on the transition energy was analyzed in detail.

First-Principles Analysis of the Effects of Covalency and Ionicity on the 4f-5d Transition Energy of Ce3+ in Garnet-Type Oxides

Abstract A blue light-emitting diode (LED) and a yellow phosphor are frequently combined to create white LEDs, with (Ce3+)-doped YAG as a common phosphor utilized in this process. Yellow light is produced when Ce3+ ions, excited by blue LEDs, are combined with direct blue light to form white light. This study investigated the effects of electronic characteristics, such as covalency and ionicity, on the 5d and 4f level energies of Ce3+ in garnet-type crystals using relativistic discrete variational-Xα (DV-Xα) molecular orbital (MO) calculations. The purpose of this study is to elucidate a detailed mechanism for the centroid shift of the 5d level energies of Ce3+ in crystals based on the MO theory. The theoretical 4f-5d transition energies agreed well with experimental ones, and electronic structure analysis revealed a high correlation between the centroid shift and the net charge of Ce3+. The primary cause of the centroid shift of the 5d level energies relative to the lowest 4f level of Ce3+ in crystals is an increase of the 4f level energies caused by a reduction of the net charge of Ce3+. These results provide a theoretical foundation for the creation of novel Ce3+-doped garnet phosphors for use in displays and solid-state lighting.

Study on Local-Structure Symmetrization of K2XF6 Crystals Doped with Mn4+ Ions by First-Principles Calculations.

The crystals of Mn4+-activated fluorides, such as those of the hexafluorometallate family, are widely known for their luminescence properties. The most commonly reported red phosphors are A2XF6: Mn4+ and BXF6: Mn4+ fluorides, where A represents alkali metal ions such as Li, Na, K, Rb, Cs; X=Ti, Si, Ge, Zr, Sn, B = Ba and Zn; and X = Si, Ge, Zr, Sn, and Ti. Their performance is heavily influenced by the local structure around dopant ions. Many well-known research organizations have focused their attention on this area in recent years. However, there has been no report on the effect of local structural symmetrization on the luminescence properties of red phosphors. The purpose of this research was to investigate the effect of local structural symmetrization on the polytypes of K2XF6 crystals, namely Oh-K2MnF6, C3v-K2MnF6, Oh-K2SiF6, C3v-K2SiF6, D3d-K2GeF6, and C3v-K2GeF6. These crystal formations yielded seven-atom model clusters. Discrete Variational Xα (DV-Xα) and Discrete Variational Multi Electron (DVME) were the first principles methods used to compute the Molecular orbital energies, multiplet energy levels, and Coulomb integrals of these compounds. The multiplet energies of Mn4+ doped K2XF6 crystals were qualitatively reproduced by taking lattice relaxation, Configuration Dependent Correction (CDC), and Correlation Correction (CC) into account. The 4A2g→4T2g (4F) and 4A2g→4T1g (4F) energies increased when the Mn-F bond length decreased, but the 2Eg → 4A2g energy decreased. Because of the low symmetry, the magnitude of the Coulomb integral became smaller. As a result, the decreasing trend in the R-line energy could be attributed to a decreased electron-electron repulsion.

Structure of Aqueous Scandium(III) Nitrate Solution by Large-Angle X-ray Scattering Combined with Empirical Potential Refinement Modeling, X-ray Absorption Fine Structure, and Discrete Variational Xα Calculations

Abstract Large-Angle X-ray scattering (LAXS) and Sc K X-ray absorption near-edge structure (XANES) measurements are made at room temperature on a 1 M (= mol dm−3) Sc(NO3)3 aqueous solution. The X-ray interference function is subjected to an empirical potential structure refinement (EPSR) modeling to extract the site-site pair correlation functions, the coordination number distribution, and the spatial density functions (three-dimensional structure). The LAXS analysis combined with EPSR reveals that Sc3+ is surrounded by six or seven water molecules and one oxygen atom of NO3− with an Sc3+-Ow (H2O) and Sc3+-ON (NO3−) distance of 2.14 Å. NO3− has a weak solvation shell with a broad distribution of coordination numbers with the highest population of nine at an N-Ow distance of 3.66 Å. Solvent water forms the tetrahedral network structure. The XANES spectrum is compared with simulated spectra on various coordination geometries of an ScO7 moiety with a discrete variational Xα (DV-Xα) molecular orbital (MO) method. A distorted monocapped trigonal prism structure of Sc3+ best reproduced the experimental pre-peak due to the Sc 1s → 3d transition in the XANES spectrum.

First-Principles calculations of the interconfigurational transition energies of 4f - 4f-15d of Ln3+ ions in LiYF4 and CaF2

In studying the design of novel phosphors and solid-state laser materials, luminescence with a characteristically long lifetime originates from f - f emissions (infrared, visible and ultraviolet). However, due to high demand for novel luminescence materials which excite in the ultraviolet (UV) and vacuum ultraviolet (VUV) ranges, the interconfigurational f - d transition has been of great interest. In this work, we investigated the 4 f n - 4 f n -1 5 d transition energy of trivalent lanthanide ions in LiYF 4 and CaF 2 host crystals (LiYF 4 : Ln 3+ and CaF 2 : Ln 3+ ) without referring to any empirical parameters. These crystals are considered to be promising fluoride hosts for solid-state laser crystals. LnF 8 5− atomic clusters were built based on the crystal structures of LiYF 4 and CaF 2 . The relativistic MO calculations were done using the relativistic Discrete Variational Xα (DV-Xα) algorithm. The relativistic version of discrete variational multi-electron method (DVME) algorithm was used to calculate the multiplet energies. We considered the effect of lattice relaxation by a simple estimation using Shannon's crystal radii. Given that the theoretical transition energies are typically underestimated due to the underestimation of the corresponding effects, the absolute transitional energies have received corrections based on one-electron calculations utilizing the Slater's transition-state technique. The in-depth study indicates that the lattice-relaxation effect does not substantially impact LiYF 4 : Ln 3+ , but greatly affects the transitional energy of heavy lanthanides doped in CaF 2 from 4 f n - 4 f n -1 5 d . The energy corrections based on Slater's transition-state method are necessary to improve the accuracy of our calculations. By a combination of these two methods, the transition energies of 4 f n - 4 f n -1 5 d of LiYF 4 : Ln 3+ and CaF 2 : Ln 3+ agree very well with the experimental data. This research is important to providing guidance in the search for new lanthanide-based phosphors in UV or VUV regions. • The 4 f n - 4 f n -1 5 d transition energies of LiYF 4 :Ln 3+ and CaF 2 :Ln 3+ have been studied. • Effect of lattice relaxation was simply estimated based on Shannon's crystal radii. • Energy corrections utilizing Slater's transition-state method were considered. • The lattice relaxation greatly affects the transitional energy of heavy CaF 2 :Ln 3+ . • The energy corrections are necessary to improve the accuracy.

Chromaticity coordinates of ruby based on first-principles calculation

Understanding the local atomic configuration is crucial for studying phosphor materials. Their performance in many applications is strongly dependent upon their optical properties. Much experimental and theoretical effort has been made to meet the requirements. Specifically, ab-initio studies have extensively reported the absorption spectra and the multiplet energies of phosphors. However, the qualitative analysis on the emitted light has not yet been reported. In this work, we characterized the emitted light of ruby, which is a widely studied phosphor material. The absorption spectra of ruby were calculated utilizing the non-empirical discrete variational Xα (DV-Xα) and discrete variational multi-electron (DVME) software. Then, the investigation on the ( x , y ) chromaticity coordinates of was performed under the standard illuminant D65 utilizing ColorAC software, a chromaticity diagram maker. In this work, we used a ruby model cluster generated from an α-Al 2 O 3 crystal. The model consists of seven atoms, where one chromium atom surrounded by six oxygen atoms. We compared the absorption spectra obtained via simple configuration interaction (CI) and those obtained which include energy corrections called configuration dependent correction (CDC) and correlation correction (CC). We successfully reproduced the color that is observed in experiment. The chromaticity coordinates approach red region for higher concentration. The results show that the calculation with CDC-CC shows better agreement with experiment. This research confirms the non-empirical calculations based on the DV-Xα and DVME methods, in the terms of emitted light. • The emitted light of α-Al2O3: Cr3+ (ruby) was characterized non-empirically. • Effects of CDC-CC correction were investigated thoroughly. • The “theoretical color” reproduced the tendency of the “experimental color”. • The calculation with CDC-CC correction shows better agreement.

(Luminescence and Display Materials Division Centennial Outstanding Achievement Award Address) Investigation of Luminescent Materials based on First-principles Calculations of Multiplet States

The transitions between multiplet states of rare-earth (RE) and transition-metal (TM) ions in crystals are utilized in variety of luminescent materials. Therefore, for the theoretical design of novel luminescent materials, a deep understanding of multiplet states and a nonempirical prediction of multiplet spectra are indispensable. For these purposes, a configuration interaction calculation program based on the discrete-variational Xa (DV-Xa) molecular orbital method [1] was developed, which is known as the discrete-variational multi-electron (DVME) program [2]. There is also a relativistic version of the DVME program [3] based on the relativistic DV-Xa method [4], which is especially useful for the analysis of RE-doped luminescent materials [5] since both the relativistic effects and the many-electron effects can be considered simultaneously without any empirical parameters. The DVME calculations have been applied to analyze various luminescent materials such as ruby [2], Mn4+-doped fluorides [6] and RE-doped LiYF4 [5]. Various types of multiplet spectra originating from d-d, f-f, f-d, and charge transfer (CT) [7, 8] transitions have been reproduced without any empirical parameters and the origins of the spectral peaks have been elucidated based on the configuration analysis of the explicitly obtained many-electron wave functions. In addition, relationships between the multiplet energy and the local structure of TM ions in crystals have been investigated in detail by creating various energy-structure maps based on systematic DVME calculations. The systematic data obtained by first-principles calculations can be also used as the training data for machine learning to create a simple predictive model. In this lecture, some representative results of the investigation of luminescent materials based on first-principles calculations of multiplet states using the DVME program will be presented.[1] H. Adachi, M. Tsukada, and C. Satoko, J. Phys. Soc. Jpn., 45, 875 (1978).[2] K. Ogasawara T. Ishii, I. Tanaka, and H. Adachi, Phys. Rev. B, 61 143 (2000).[3] K. Ogasawara, T. Iwata, Y. Koyama, and T. Ishii, I. Tanaka, and H. Adachi, Phys. Rev. B, 64, 115413 (2001).[4] A. Rosén, D.E. Ellis, H. Adachi, and F.W. Averill, J. Chem. Phys., 65, 3629 (1976).[5] K. Ogasawara, S. Watanabe, H. Toyoshima, M.G. Brik, Handbook on Physics and Chemistry of Rare Earths, 37, 1 (2007).[6] M. Novita, T. Honma, B. Hong, A. Ohishi, K. Ogasawara, J. Lumin. 169, 596 (2016).[7] S. Takemura, K. Ogasawara, Opt. Mater.: X, 1, 100005 (2019).[8] S. Takemura, K. Ogasawara, J. Lumin. 214, 116542 (2019).

Study on the Optical Luminescence Properties of Li2TiO3: Mn4+ and Cr3+

Abstract We studied the optical luminescence properties of Li2TiO3: Mn4+ based on first-principles calculations. The optical luminescence properties such as Mn-O distance, molecular orbital (MO) energy, crystal field splitting, multiplet energy levels, and Coulomb interaction are discussed in detail. The impact of lattice relaxation was examined through geometry optimizations based on first-principles band-structure calculations using Cambridge Serial Total Energy Package (CASTEP) software. Both Discrete Variational-Xα (DV-Xα) and the Discrete Variational Multi-electron (DVME) software were used to estimate the optical luminescence properties. Here we also predicted the optical luminescence properties of Li2TiO3: Cr3+. The results imply that the multiplet energy levels tend to decline in the order of Mn4+ to Cr3+. The integral Coulomb's decreasing tendency is in excellent agreement with the declining tendency of 2E, 2T1, 2T2 energies of Li2TiO3: Mn4+ and Li2TiO3: Cr3+.

Study on the molecular orbital energies of ruby under pressure

We calculated the molecular orbital (MO) energies of α-Al2O3: Cr3+ (ruby) under pressure non-empirically based on one-electron calculations. The pressure was applied to the samples from ca. 0–110 GPa. Two different approaches to estimate the effect of lattice relaxation are Shannon's crystal radii method and the geometry optimization using CASTEP software. The one-electron calculations were used to estimate the MO energies, which were performed using the Discrete Variational Xα (DV-Xα) software. We found that the splitting of the crystal field (10Dq) increased as the applied pressure grew. We pointed out that collectively considering the effect of lattice relaxation and using the more substantial cluster size has a significant effect on the value of 10Dq.

Recent applications of discrete variational multi-electron method

The discrete variational multi-electron (DVME) method is a first-principles many-electron calculation method based on the discrete-variational Xα (DV-Xα) molecular orbital method. There is also the relativistic version of the DVME method, which is based on the relativistic DV-Xα method. Since the explicit many-electron wave functions of the multiplet states of transition-metal (TM) or rare-earth (RE) ions in crystals can be obtained, the DVME method has been applied to the analyses of various luminescent materials such as solid-state lasers and phosphors. In this paper, as examples of the recent applications of the DVME method, the following researches will be briefly presented: (1) Visualization of the energy-structure relationship of Cr3+ in oxides, and (2) Creation of the predictive model of the multiplet energy using machine learning.

Optical properties of Co3+ doped in α-Al2O3 with Considering Lattice Relaxation Effect

Search of new luminescence materials to improve the current white-ight emitting diode (LED) is still ongoing. Several investigations such as the ion dependence, the host crystal dependence has been done previously. Since we want to get a complete picture of ion-host combination dependence, we investigated α-Al2O3: TM3+. In this work we estimated the optical properties of α-Al2O3 doped with Co3+. We constructed model clusters consisting of 7 atoms. The lattice relaxation effects due to the Co3+ substitution were calculated using Shannon’s crystal radii method and geometry optimizations in the Cambridge Serial Total Energy Package (CASTEP) method. The one-electron Discrete Variational-Xα (DV-Xα) method was used to estimate the molecular orbital energies, while the many-electron Discrete Variational Multielectron (DVME) method was used to estimate the d-d absorption spectra. Since Co3+ belongs to 3d 6 configuration, there are two possible spin states i.e., high-spin (HS) and low-spin (LS) states.

Effect of trivalent manganese substitution in α-Al2O3 crystal on the absorption spectra based on first-principles calculations

The absorption spectra of α-Al2O3: Mn3+ at low-spin (LS) state have been estimated based on first-principles calculations without referring to any experimental parameter. The effect of lattice relaxation due to the Mn3+ substitution was considered under several computational procedures such as the Shannon’s crystal radii method and the geometry optimization using Cambridge Serial Total Energy Package (CASTEP) code. Two different sizes of model clusters consisting of 7 and 63 atoms were compared. Next, the molecular orbitals (MO) were estimated using the one-electron calculations discrete variational-Xα (DV-Xα) method while the absorption spectra were estimated using the many-electron calculations discrete variational multi-electron (DVME) method. The results show that the lattice-relaxation ratio is about ca. 104.05-106.43% depending on the computational conditions. Due to the Mn-O bond-length elongation, the crystal field splitting (10Dq) decreased ca. 0.24-0.41 eV. Thus, the peak position originating from 3T1 → 3E transition energy shifted toward the lower energy ca. 0.10-0.26 eV. Both of the larger-cluster size and the lattice-relaxation effect decrease the peak energies.

Variations of chemical bonds in silver halides under high pressures

ABSTRACTRocksalt structured AgI (rs-AgI), which appears under pressures between 0.4 and 11.3 GPa, shows high ionic conductivity as high as that in α-AgI, especially at high temperatures. Microscopic origins of ion conduction mechanisms have not been clarified until now and are therefore investigated using the discrete variational Xα (DV-Xα) cluster method. Comparable studies for AgCl and AgBr, which are known as high ionic conductors just below melting temperatures and form the rocksalt structure, are also done. Ionic interactions between a mobile Ag ion and remaining ions are almost the same between those under different pressures, while covalent interactions between the mobile Ag ion and the remaining Ag ions change drastically when the mobile Ag ion is migrating. Similar results are also obtained for AgCl and AgBr. The covalent interactions between the mobile Ag ion and the remaining Ag ions, which should affect the Ag ion migration, play important roles in not only rs-AgI but AgBr and AgCl.

Ab-initio study on the absorption spectrum of color change sapphire based on first-principles calculations with considering lattice relaxation-effect

In this study, we performed an investigation on α-Al2O3: V3+ material, or the so-called color change sapphire, based on first-principles calculations without referring to any experimental parameter. The molecular orbital (MO) structure was estimated by the one-electron MO calculations using the discrete variational-Xα (DV-Xα) method. Next, the absorption spectra were estimated by the many-electron calculations using the discrete variational multi-electron (DVME) method. The effect of lattice relaxation on the crystal structures was estimated based on the first-principles band structure calculations. We performed geometry optimizations on the pure α-Al2O3 and with the impurity V3+ ion using Cambridge Serial Total Energy Package (CASTEP) code. The effect of energy corrections such as configuration dependence correction and correlation correction was also investigated in detail. The results revealed that the structural change on the α-Al2O3: V3+ resulted from the geometry optimization improved the calculated absorption spectra. By a combination of both the lattice relaxation-effect and the energy correction-effect improve the agreement to the experiment fact.

Theoretical study of the temperature dependent hydrogen storage capacity of Pd and Ti nanoparticles

A hybrid simulation was carried out using discrete variational (DV) Xα molecular orbital and molecular dynamics methods to examine the storage of hydrogen in metal nanoparticles. The calculation load was light and therefore could be performed quickly on a generic personal computer. The simulation investigates the electronic states of hydrogen on Pd and Ti nanoparticles. The hydrogen dissociation behavior on the metal surface was reproduced, and the calculated bonding orbitals of Pd and hydrogen are consistent with other reports. The diffusion coefficient of hydrogen inside the metal displays the same temperature dependence as the theoretical results. This simplified calculation, which produces results in agreement with experimental/theoretical values, could lead to improved simulation methods for hydrogen storage materials.