Development of Density-Functional Tight-Binding Parameters for the Molecular Dynamics Simulation of Zirconia, Yttria, and Yttria-Stabilized Zirconia.

In this work, a set of density-functional tight-binding (DFTB) parameters for the Zr-Zr, Zr-O, Y-Y, Y-O, and Zr-Y interactions was developed for bulk and surface simulations of ZrO2 (zirconia), Y2O3 (yttria), and yttria-stabilized zirconia (YSZ) materials. The parameterization lays the ground work for realistic simulations of zirconia-, yttria-, and YSZ-based electrolytes in solid oxide fuel cells and YSZ-based catalysts on long timescales and relevant size scales. The parameterization was validated for the zirconia and yttria polymorphs observed under standard conditions based on density functional theory calculations and experimental data. Additionally, we performed DFTB-based molecular dynamics (MD) simulations to compute structural and vibrational properties of these materials. The results show that the parameters can give a qualitatively correct phase ordering of zirconia, where the tetragonal phase is more stable than the cubic phase at a lower temperature. The lattice parameters are only slightly overestimated by 0.05-0.1 Å (2% error), still within the typical accuracy of first-principles methods. Additionally, the MD results confirm that zirconia and yttria phases are stable against transformations under standard conditions. The parameterization also predicts that vibrational spectra are within the range of 100-1000 cm-1 for zirconia and 100-800 cm-1 for yttria, which is in good agreement with predictions both from full quantum mechanics and a recently developed classical force field. To further demonstrate the advantage of the developed DFTB parameters in terms of computational resources, we conducted DFTB/MD simulations of the YSZ4 and YS12 models containing approximately 750 atoms.

Density-Functional Tight-Binding Parameters for Bulk Zirconium: A Case Study for Repulsive Potentials.

Density-functional tight-binding (DFTB) parameters are presented for the simulation of the bulk phases of zirconium. Electronic parameters were obtained using a band structure fitting strategy, while two-center repulsive potentials were created by particle swarm optimization. As objective functions for the repulsive potential fitting, we employed the Birch-Murnaghan equations of state for hexagonal close-packed (HCP), body-centered cubic (BCC) and ω phases of Zr from density-functional theory (DFT). When fractional atomic coordinates are not allowed to change in the generation of the equation-of-state curves, long-range repulsive DFTB potentials are able to almost perfectly reproduce equilibrium structures, relative DFT energies of the bulk phases, and bulk moduli. However, the same potentials lead to artifacts in the DFTB potential energy surfaces when atom positions in the unit cell are allowed to fully relax during the change of unit cell parameters. Conventional short-range repulsive DFTB potentials, while inferior in their ability to reproduce DFT bulk energetics, are able to correctly reproduce the qualitative shape of the DFT potential energy surfaces, including the location of global minima, and can therefore be considered more transferable.

Investigation of the chemical and optical properties of halogen-substituted N-methyl-4-piperidone curcumin analogs by density functional theory calculations.

Halogen-substituted N-methyl-4-piperidone curcumin analog compounds were studied for their molecular structure, molecular vibration analysis, natural bond orbital, ultraviolet spectra, molecular reactivity analysis, and nonlinear optical properties. The molecules in interest were the (3E,5E)-3,5-dibenzylidene-1-methylpiperidin-4-one along with its substituted halogen (X = F- or Cl- or Br-) at the ortho, meta and para position in the phenyl ring. The calculated geometrical parameters at B3LYP/6-311++G(d,p) were in good agreement with the available experimental XRD data. Using the frequency calculations, the molecular vibrational modes have been analyzed. Meanwhile, the hyperconjugative stabilization energies have been calculated using NBO analysis to address the donor and acceptor in the hyperconjugation. The calculated UV spectra at TD-CAM-B3LYP/6-311++G(d,p) show strong absorption around 300 to 320 nm which corresponds to the absorption in the UV-A and UV-B region. From molecular electrostatic potential surface, CO moiety is one of the electronegative regions in the compounds where the CX moiety is the other electronegative region of the compounds. Fluoro-substituted compounds are the compounds with the most electronegativity while bromo-substituted compounds are the compounds with the least electronegativity. Calculations of average local ionization energy surfaces have been performed to obtain information related to the local reactivity of the molecules where the phenyl becomes less reactive after the substitution. The calculated first hyperpolarizability is 6 to 17 times larger than urea, the standard nonlinear optic material. These findings imply that all of the molecules considered have potential to be applied as active sunscreen material or nonlinear optic material.