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In this work, we will investigate structural, electronic, magnetic, and thermodynamic properties using density functional theory (DFT) and the quasi-harmonic Debye model. We consider ferromagnetic (FM) and non-magnetic (NM) states for L2₁ and Hg₂CuTi-type crystal structures. The best stability is obtained for ferromagnetic Rh₂MnGa in a Cu₂MnAl structure with a lattice parameter of 6.07 Å and a total magnetic moment of 4.11${\mu }_B$. The compressive strain range from $-6$% to $+4$% tensile strain maintains the ferromagnetic nature and enhances the magnetic moment up to 4.39${\mu }_B$. The formation energy confirms the inherent stability of Rh₂MnGa. Other important thermodynamic parameters such as the expansion coefficient ($\alpha$), heat capacity ($C_V$), Debye temperature (${\theta }_D$) and Grüneisen constant ($\gamma$) are also estimated in this work.

This research is an introduction to density functional theory (DFT), which has been designed for Floating Spherical Gaussian Orbital (FSGO) method for the first time. Our principal objective is to apply a combination of energy functionals to the FSGO densities. The functionals used are separated into exchange and correlation parts. For the exchange part the Becke exchange that includes gradient correction is used. The correlation part has been carried out using Lee, Yang and Parr gradient-corrected functional. Three goals are investigated in this research. Is it possible to apply DFT in the FSGO procedure to obtain the electronic structure of chemical species? Second, is it a stable condition, from the variational point of view, during optimization of exponents and coefficients of each Gaussian? Thirdly, when the two above questions are encouraging, are the results consistent with other results in the literature? In this research we are looking for acceptable answers to the above questions.

In this work we introduce a hybrid ab initio-classical simulation methodology designed to incorporate the chemistry into the description of phenomena that, intrinsically, require very large systems to be properly described. This hybrid approach allows us to conduct large-scale atomistic simulations with a simple classical potential (embedded atom method (EAM), for instance) while simultaneously using a more accurate ab initio approach for critical embedded regions. The coupling is made through shared atomic shells where the two atomistic modeling approaches are relaxed in an iterative, self-consistent manner. The magnitude of the incompatibility forces arising in the shared shell is analyzed, and possible terminations for the embedded region are discussed, as a way to reduce such forces. As a test case, the formation energy of a single vacancy in aluminum at different distances from an edge dislocation is studied. Results obtained using the hybrid approach are compared to those obtained using classical methods alone, and the range of validity for the classical approach is evaluated.

The first-principles calculations based on Density Functional Theory (DFT) within generalized gradient approximation (GGA) of Engel–Vosko–Perdew–Wang and modified exact exchange potential of Becke–Johnson have been introduced for the structural and electronic properties of the Sc_xAl_1-xN alloys, respectively. The present lattice constants calculated for the ScAlN alloys and the end compounds (AlN and ScN) are found to be in very good agreement with the available experimental and theoretical ones. The stable ground state structures of the Sc_xAl_1-xN alloys are determined to be wurtzite for the Sc concentration less than ~0.403 and rock-salt for the higher Sc concentrations. The present electronic band structure calculations within Becke–Johnson scheme are found to be capable of providing energy band gaps of the AlN and ScN compounds very close to the ones of the available experiments and expensive calculations. According to the calculations of Becke–Johnson potential, the Sc_xAl_1-xN alloys in the wurtzite and zinc-blende structures are direct band gap materials for the Sc concentrations in the ranges of (0.056 ≤ x ≤ 0.833) and (0.03125 ≤ x ≤ 0.0625, 0.375 ≤ x ≤ 0.96875), respectively. However, the ScAlN alloys in the rock-salt phase are determined to be direct band gap materials for total range of the Sc concentration considered in this work. While the energy gaps of the RS-AlScN alloys are found to be extending from near ultraviolet to near infrared with a large (negative) bowing, the ones of the WZ-AlScN and ZB-AlScN alloys are determined to be varying in a small energy range around near ultraviolet with a small (negative) bowing.

Structural stability and electronic properties of GaAs${}_{1-x}$P${}_x$ ($0.0\le x\le 1.0$) nanowires (NWs) in zinc-blende (ZB) ($\sim 5\le$ diameter $\le \sim 21$Å) and wurtzite (WZ) ($\sim 5\le \mathrm{\text{diameter}}\le \sim 29$Å) phases are investigated by first-principles calculations based on density functional theory (DFT). GaAs ($x=0.0$) and GaP ($x=1.0$) compound NWs in WZ phase are found energetically more stable than in ZB structural ones. In the case of GaAs${}_{1-x}$P${}_x$ alloy NWs, the energetically favorable phase is found size and composition dependent. All the presented NWs have semiconductor characteristics. The quantum size effect is clearly demonstrated for all GaAs${}_{1-x}$P${}_x$ ($0.0\le x\le 1.0$) NWs. The band gaps of ZB and WZ structural GaAs compound NWs with $\sim 10\le$ diameter $\le \sim 21$Å and $\sim 5\le \mathrm{\text{diameter}}\le \sim 29$Å, respectively are enlarged by the addition of concentrations of phosphorus for obtaining GaAs${}_{1-x}$P${}_x$ NWs proportional to the x values around 0.25, 0.50 and 0.75.

The adsorption behavior and electronic properties of CO and O₂ molecules at the supported Pt and Eu atoms on (5,5) armchair SWCNT have been systematically investigated within density functional theory (DFT). Fundamental aspects such as adsorption energy, natural bond orbital (NBO), charge transfer, frontier orbitals and the projected density of states (PDOS) are elucidated to analyze the adsorption properties of CO and O₂ molecules. The results reveal that B- and N-doping CNTs can enhance the binding strength and catalytic activity of Pt (Eu) anchored on the doped-CNT, where boron-doping is more effective. The electronic structures of supported metal are strongly influenced by the presence of gases. After adsorption of CO and O₂, the changes in binding energy, charge transfer and conductance may lead to the different response in the metal-doped CNT-based sensors. It is expected that these results could provide helpful information for the design and fabrication of the CO and O₂ sensing devices. The high catalytic activity of Pt supported at doped-CNT toward the interaction with CO and O₂ may be attributed to the electronic resonance particularly among Pt-5d, CO-2$\pi$* and O₂-2$\pi$* antibonding orbitals. In contrast to the supported Eu at doped-CNT, the Eu atom becomes more positively charged, which leads to weaken the CO adsorption and promote the O₂ adsorption, consequently enhancing the activity for CO oxidation and alleviating the CO poisoning of the europium catalysts. A notable orbital hybridization and electrostatic interaction between these two species in adsorption process being an evidence of strong interaction. The electronic structure of O₂ adsorbed on Eu-doped CNT resembles that of O${}_2^{-}$, therefore the transferred charge weakens the O–O bonds and facilitates the dissociation process, which is the precondition for the oxygen reduction reaction (ORR).

In this work, a detailed study of the structural, electronic and absorption properties of crystalline 2,6-dimethyl-4-(diphenylmethylene)-2,5-cyclohexadienone with $\alpha$ form ($\alpha$-DDCD) in the pressure range of 0–250GPa is performed by density-functional theory (DFT) calculations. The particular analysis of the variation tendencies of the lattice constants, bond lengths and bond angles under different pressures shows that there occur complex transformations in $\alpha$-DDCD under compression. In addition, it can be see that the $b$-direction is much stiffer than the $a$- and $c$-axes in the structure of $\alpha$-DDCD, suggesting the compressible crystal of $\alpha$-DDCD has anisotropy. Then, by analyzing the bandgap and density of states (DOS) of $\alpha$-DDCD, it is found that the crystal undergoes a phase transformation from semiconductor to metal at 90GPa and it becomes more sensitive under compression. Besides, in the pressure range 110–170GPa, repeated transformations between metal and semiconductor occur four times, suggesting the structural instability of $\alpha$-DDCD in this pressure range. Finally, the relatively high optical activity with the pressure increases of $\alpha$-DDCD is seen from the absorption spectra, and two obvious structural transformations are also observed at 130GPa and 140GPa, respectively.

In this work, we use density functional theory (DFT) calculations to study the structural, electronic and absorption properties of crystalline 2-benzylidene-1-indanone (signed as 2-BI) in the pressure range of 0–300GPa. The detailed analysis of the variation tendencies of the lattice constants, bond lengths and bond angles with increasing pressures shows that there occur several transformations in 2-BI under different pressures. In addition, it can be see that the $a$- and $c$-axis are much stiffer than the $b$-axis in the structure of 2-BI, suggesting the crystal is anisotropic. Then, the analysis of the band gap and DOS (PDOS) of 2-BI indicate that its electronic character has changed at 120GPa into metal phase, but then transfer into excellent insulator at 230GPa. Moreover, the relatively high optical activity with the increasing pressure of 2-BI is seen from the absorption spectra, and three obvious structural transformations are also observed at 60, 120 and 250GPa, respectively.

In this work, a detailed study of the structural, electronic and optical absorption properties of crystalline benzoic acid in the pressure range of 0–300GPa is performed by density functional theory (DFT) calculations. We found that occur complex transformations in benzoic acid under compression occurs, by analyzing the variation tendencies of the lattice constants, bond lengths and bond angles under different pressures. In the pressure range 0–280GPa, repeated formations and disconnections of hydrogen bonds between H1(P1) atom and O1(P1), O2(P4-$x$-$y$-$z$) atoms occur several times, and a new eight-atom ring (benzoic acid dimer) forms at 100GPa and 280GPa. Then, by analyzing the band gap and density of states (DOS) of benzoic acid, it is found that the crystal undergoes a phase transformation from insulator to semiconductor at 240GPa and it even becomes metal phase at 280GPa. In addition, the relatively high optical activity with the pressure increases of benzoic acid is seen from the absorption spectra, and three obvious structural transformations are also observed at 110, 240 and 290GPa, respectively.

In this work, a detailed study of the structural, electronic and absorption properties of crystalline 1-ethyl-1,4-dihydro-7-methyl-4-oxo-1,8-naphthyridine-3-carboxylic acid (nalidixic acid) in the pressure range 0–300GPa is performed by density functional theory (DFT) calculations. The detail analysis of the variation tendencies of the lattice constants, bond lengths and bond angles with increasing pressures shows that complex transformations occur in nalidixic acid under compression. In addition, it can be see that the $a$- and $c$-axes are much stiffer than the $b$-direction in the structure of nalidixic acid, suggesting the crystal is anisotropic. In the pressure range 90–250GPa, repeated formations and disconnections of covalent bonds between C6 (P1 or P4) and O1 (P4 or P1) occur several times, and a new eight-atom ring forms at 90, 160, 190 and 230GPa, respectively. Then, the analysis of the bandgap and density of states (DOS) of nalidixic acid indicates that its electronic character changes at 230GPa into an excellent insulator, but the electron transition is much easier at several pressure regions for the bandgap closing to 0eV. Moreover, as the pressure increases relatively high optical activity of nalidixic acid is seen from the absorption spectra, and two obvious structural transformations are also observed at 200 and 230GPa, respectively.

This is an ab initio study based on the density functional theory that uses GGA-PBE as the exchange–correlation potential. The energetic, electronic, magnetic properties, and optical conductivity of the cubic $\beta 2$ of TiCo and TiNi alloys with and without the hydrogen atom are performed. The present alloys are found to be thermodynamically stable and can be created. It can be deduced that the octahedral site has higher energetic stability absorption for the hydrogen atoms compared to the bridge and tetrahedral sites in the TiCo and TiNi alloys. The absorption energy at octahedral site is found to be 2.37eV for TiCo and 2.32eV for TiNi. Hydrogen absorption expands and brittles the host alloy. Hydrogen storage in more than one site in the host alloy is found to be energetically stable and can be formed. The chemical bonding between the constituent atoms of the present alloys is mainly ionic with some covalent bonding. The hydrogen absorption has a clear effect on the magnetic, and electrical conductivity relative to the relaxation time and optical conductivity of the present alloys. Beneficial optical applications can be assumed for the present alloys due to their high optical conductivity.

In this paper, the structural, electronic and absorption properties of 2,2′-iminobis (acetamide oxime) (IBO) under pressure of 0–300GPa are calculated by the density functional theory (DFT) calculations. Analysis of the variation trend of lattice constant, bond length and bond angle of IBO under compression conditions, shows there are complex transformations under different pressure. In addition, it is found that the structure of IBO in the $a$-axis is stiffer than that along the $b$- and $c$-axes, which indicates that the crystal has anisotropic compressibility. By analyzing the band structure and the density of states of IBO, it is seen that at 120GPa, the electronic structure of IBO changes into metallic system, and becomes more sensitive under compression conditions. The transition between metal and semiconductor occurs again at 150Gpa. Finally, at 180GPa, the crystal transforms into metal again. The three obvious phase transitions indicate that the structure of IBO becomes more unstable with the increase of pressure. The absorption spectra show that with the increase of pressure, the optical activity of IBO crystal grows higher, and three obvious structural transitions are, respectively, observed at 120, 150 and 180GPa.

In this work, the structural, electronic and absorption properties of 2-methyl-2H-naphtho-[1,8-de]triazine in the pressure ranges of 0–250GPa are studied in detail (hereinafter referred to as 2-methyl crystal). Density functional theory (DFT) is used to calculate the lattice constants, bond lengths and bond angles of 2-methyl under different pressures. The results show that the crystals undergo complex transformations under compression, and the major structural transformations occur at pressures of 90GPa and 210GPa with repeated formations and disconnections. In addition, the $a$- and $c$-directions of the 2-methyl are stiffer than the $b$-direction, which indicates that the compressibility of the crystal is anisotropic. From the specific analysis of the bandgaps of 2-methyl, we can know that the crystal is converted from semiconductor to metal at 90GPa. The absorption spectrum of the crystal also indicates that 2-methyl has a relatively high optical activity with the increasing pressure.

In this work, detailed DFT calculations of the structural, mechanical and electronic properties of crystalline CaSi₂ with four different structures in the pressure range of 0–50 GPa are performed by GGA-PBE. It is found that the Enthalpy differences imply that the R$\bar{3}$m phase $\to$ I4₁/amd phase $\to$ P6/mmm phase transition in CaSi₂ occur at $P_1=2.5$GPa, $P_2=33.5$GPa by using the XC of GGA, which is consistent with previous experiments and theoretical conclusions. Besides, the elastic stability criterion is used to study the change of the elastic constant of CaSi₂ under pressures. In particular, the bulk modulus $B$, shear modulus $G$, Young’s modulus $E$, sound velocity $v$ and brittleness and toughness properties of CaSi₂ under pressures are comprehensively studied for the first time. Finally, the changes of the anisotropy factor of CaSi₂ are studied under different pressures, and the electronic structure is studied in detail.

In this paper, the structural, electronic and optical absorption properties of $m$-aminobenzoic acid crystals (hereinafter referred to as $m$-amino) in the pressure range of 0–300GPa are calculated by density functional theory (DFT). The changing trend of the lattice constant of $m$-amino under different pressures is analyzed. We find that the crystal undergoes complex transformation. Furthermore, it can be seen that the structure of $m$-amino along the $b$-axis is stiffer than that along the $a$-axis and $c$-axis, suggesting that the crystal has anisotropic compressibility. Through the analysis of the band gap and density of states of $m$-amino, it is found that the electronic properties of $m$-amino are transformed from semiconductor phase to metal phase at 100GPa, then jump into the semiconductor phase and maintain the metal phase again in the pressure range of 150–250GPa. Repeated phase transitions indicate that the structure of $m$-amino becomes more unstable as the pressure increases. Besides, from the absorption spectra, the optical activity of $m$-amino is relatively high with the increase of pressure, and two obvious structural transitions are observed at 70 and 270GPa, respectively.

We investigate the electronic structure and the optical characterizations of iron incorporating titanium dioxide by ab initio method by employing the density functional theory. We show that Fe atoms can be incorporated into TiO₂ by replacing Ti atoms in the crystal structure of ${\mathrm{\text{Fe}}}_x{\mathrm{\text{Ti}}}_{1-x}{\mathrm{\text{O}}}_2$ according to the ratio $X=0.00$, 0.25, 0.50, 0.75 and 1.00. The partial density of state and the energy band structure of the optimized structure have been calculated. Adding Fe atoms to the TiO₂ causes shrinkage of the bands in the band structure that leads to a decrease in the energy gap of the pure titanium dioxide crystal structure. The results of the optical properties showed that the titanium dioxide has no absorption in the range of the visible light and detect only in the ultraviolet light (UV). The optical constant absorption coefficient, reflectivity and real and imaginary parts of the dielectric constant have been calculated. It exhibits that the properties of pure TiO₂ will change by adding the Fe atoms to the structure, which leads to a significant enhancement in the optical characteristics.

In this work, a detailed study of the structural, mechanical and thermodynamic changes in CaZn₂ under pressure range of 0–40 GPa is performed, and two different exchange correlations (XC) of GGA-PBE and LDA are used. It is found that the enthalpy values imply that CaZn₂ undergoes a phase transition from CeCu₂-type phase to Laves phase at 3.45 GPa with XC of GGA-PBE and 1.43 GPa with LDA, which is consistent with previous experiments and parts of theoretical conclusions. Furthermore, the elastic stability criterion is used to study the elastic constants of CaZn₂ under different pressures, confirming the stability of the CaZn₂ structure from 0 GPa to 40 GPa. Finally, the thermodynamic properties of CaZn₂ under pressures are comprehensively studied in detail.

In this paper, density functional theory (DFT) is used to study the structure, electron and absorption properties of 6-Amino crystal in the pressure range of 0–300GPa. The variations of the lattice constants, bond lengths and bond angles show that they undergo complex transformations under different pressures. More narrowly, the covalent bonds of the benzene ring and the uracil ring in the planar intramolecular structure are broken, and then the new covalent bonds between the adjacent intermolecular structure are reshaped at about 80, 100 and 160GPa. The analysis results of band gap and DOS of 6-Amino indicate that the electronic properties of 6-Amino repeatedly transform from the semiconductor system into the metal phase at 80, 180 and 260GPa. The absorption spectra show two important structural changes at 100 and 180GPa, and present the high optical activity with the change of pressure.

Solid-state thermoelectric devices offer the possibility of exploiting waste heat from engines and power plants and converting it into electrical energy. One of the greatest challenges in the development of thermoelectric material systems is to find new thermoelectric materials with high thermoelectric figures of merit ZT. In this work, the structural, electronic and thermoelectric properties of PbSe${}_{1-x}$S${}_x$ ($x=0$, 0.25, 0.50, 0.75 and 1) semiconductors are investigated by applying density functional theory in a full potential linearized augmented plane wave method (FP-LAPW). Calculations of structural properties were completed using the generalized gradient approximation GGA of Perdew Burke and Ernzerhof PBE to get reliable lattice constant results with experimental values. The obtained electronic results show that the PbSe${}_{1-x}$S${}_x$ material is a narrow band gap semiconductor. In addition, the thermoelectric properties are studied on the basis of the fully iterative solution of the Boltzmann transport equation. PbSe${}_{1-x}$S${}_x$ had a high figure of merit indicating that our materials are promising candidates in thermoelectric applications.