Functional Theory Calculations Of Structures Research Articles

Structure and energetics of FeO/Fe(001) interfaces

We report results of density functional theory calculations of structure and properties of 1–5 monolayer thin FeO(001) films and their interactions with the Fe(001) surface. It is found that deposition of an iron-oxide film affects weakly geometry of the Fe(001) support, causing small 2% expansion of the first interplanar distance compared to clean iron surface. Analysis of the electronic structure of the FeO/Fe system shows that after interface formation, the oxide layer remains semiconducting and the substrate metallic. Electronic structure of the FeO(001) layer in direct contact with the Fe(001) surface exhibits metallic character. Magnetism of the metal/semiconductor interface is only slightly disturbed compared to that of isolated components. The FeO adlayers preserve antiferromagnetic (AFM) nature of the oxide and the sharp boundary between higher AFM phase of FeO and lower ferromagnetic phase of Fe is observed at the interface.

Enhanced Piezoelectric Response of AlN via CrN Alloying

Since AlN has emerged as an important piezoelectric material for a wide variety of applications, efforts have been made to increase its piezoelectric response via alloying with transition metals that can substitute for Al in the wurtzite lattice. Herein, we report density functional theory calculations of structure and properties of the Cr-AlN system for Cr concentrations ranging past the wurtzite-rocksalt transition point. By studying the different contributions to the longitudinal piezoelectric coefficient, we propose that the physical origin of the enhanced piezoelectricity in Cr$_x$Al$_{1-x}$N alloys is the increase of the internal parameter $u$ of the wurtzite structure upon substitution of Al with the larger Cr ions. Among a set of wurtzite-structured materials, we have found that Cr-AlN has the most sensitive piezoelectric coefficient with respect to alloying concentration. Based on these results, we propose that Cr-AlN is a viable piezoelectric material whose properties can be tuned via Cr composition; we support this proposal by combinatorial synthesis experiments, which show that Cr can be incorporated in the AlN lattice up to 30\% before a detectable transition to rocksalt occurs. At this Cr content, the piezoelectric modulus $d_{33}$ is approximately four times larger than that of pure AlN. This finding, combined with the relative ease of synthesis, may propel Cr-AlN as the prime piezoelectric material for applications such as resonators and acoustic wave generators.

Ab initio design of drug carriers for zoledronate guest molecule using phosphonated and sulfonated calix[4]arene and calix[4]resorcinarene host molecules

Monomolecular drug carriers based on calix[n]-arenes and -resorcinarenes containing the interior cavity can enhance the affinity and specificity of the osteoporosis inhibitor drug zoledronate (ZOD). In this work we investigate the suitability of nine different calix[4]-arenes and -resorcinarenes based macrocycles as hosts for the ZOD guest molecule by conducting {\it ab initio} density functional theory calculations for structures and energetics of eighteen different host-guest complexes. For the optimized molecular structures of the free, phosphonated, sulfonated calix[4]-arenes and -resorcinarenes, the geometric sizes of their interior cavities are measured and compared with those of the host-guest complexes in order to check the appropriateness for host-guest complex formation. Our calculations of binding energies indicate that in gaseous states some of the complexes might be unstable but in aqueous states almost all of the complexes can be formed spontaneously. Of the two different docking ways, the insertion of ZOD with the \ce{P-C-P} branch into the cavity of host is easier than that with the nitrogen containing heterocycle of ZOD. The work will open a way for developing effective drug delivering systems for the ZOD drug and promote experimentalists to synthesize them.

Molecular Design Parameters of Anthraquinone Dyes for Guest–Host Liquid-Crystal Applications: Experimental and Computational Studies of Spectroscopy, Structure, and Stability

A set of five anthraquinone dyes with bis(4-propylphenyl) substituent groups, connected via sulfide or amine linkages at the 1,5-positions or directly at the 2,6-positions, have been studied in solution by UV–vis spectroscopy and electrochemistry, allied with density functional theory calculations of structures, electronic transitions, and redox potentials. The visible transitions and redox potentials are shown to vary with the HOMO and LUMO energies, with the variation in both color and redox stability between the dyes being attributable principally to variations in the HOMOs located mainly on the substituents and outer anthraquinone rings. The calculated molecular structures and visible transition dipole moments are shown to vary subtly with substituent, giving variations in the molecular aspect ratios, minimum moment of inertia axes, and transition dipole moment vector orientations that can rationalize the alignment trends reported in the literature for such anthraquinone dyes in liquid crystal hosts, ...

Modified Corrections for London Forces in Solid-State Density Functional Theory Calculations of Structure and Lattice Dynamics of Molecular Crystals

Dispersion forces are critical for defining the crystal structures and vibrational potentials of molecular crystals. It is, therefore, important to include corrections for these forces in periodic density functional theory (DFT) calculations of lattice vibrational frequencies. In this study, DFT was augmented with a correction term for London-type dispersion forces in the simulations of the structures and terahertz (THz) vibrational spectra of the dispersion-bound solids naphthalene and durene. The parameters of the correction term were modified to best reproduce the experimental crystal structures and THz spectra. It was found that the accurate reproduction of the lattice dimensions by adjusting the magnitude of the applied dispersion forces resulted in the highest-quality fit of the calculated vibrational modes with the observed THz absorptions. The method presented for the modification of the dispersion corrections provides a practical approach to accurately simulating the THz spectra of molecular crystals, accounting for inherent systematic errors imposed by computational and experimental factors.

Ion Induced Dipole Clusters Hn–(3 ≤n-odd ≤ 13): Density Functional Theory Calculations of Structure and Energy

We investigate anew the possible equilibrium geometries of ion induced dipole clusters of hydrogen molecular ions, of molecular formula H(n)(-) (3 ≤ n-odd ≤ 13). Our previous publications [Sapse, A. M.; et al. Nature 1979, 278, 332; Rayez, J. C.; et al., J. Chem. Phys. 1981, 75, 5393] indicated these molecules would have a shallow minimum and adopt symmetrical geometries that accord with the valence shell electron pair repulsion (VSEPR) rules for geometries defined by electron pairs surrounding a central point of attraction. These earlier calculations were all based upon Hartree-Fock (HF) calculations with a fairly small basis of atomic functions, except for the H3(-) ion for which configuration interaction (CI) calculations were carried out. A related paper [Hirao, K.; et al., Chem. Phys. 1983, 80, 237] carried out similar calculations on the same clusters, finding geometries similar to our earlier calculations. However, although that paper argued that the stabilization energy of negative ion clusters H(n)(-) is small, vibration frequencies for the whole set of clusters was not reported, and so a definitive assertion of a true equilibrium was not present. In this paper we recalculate the energetics of the ion induced dipole clusters using density function theory (DFT) B3LYP method calculations in a basis of functions (6-311++G(d,p)). By calculating the vibration frequencies of the VSEPR geometries, we prove that in general they are not true minima because not all the resulting frequencies correspond to real values. By searching the energy surface of the B3LYP calculations, we find the true minimum geometries, which are surprising configurations and are perhaps counterintuitive. We calculate the total energy and binding energy of the new geometries. We also calculate the bond paths associated with the quantum theory of atoms in molecules (QTAIM). The B3LYP/6-311++G(d,p) results, for each molecule, deliver bond paths that radiate between each polarized H2 molecule and the polarizing H(-) ion.

Density functional theory calculations of structure, FT-IR and Raman spectra of S-phenyl thioacetate

The infrared (4000–400 cm −1) and Raman spectra (3700–100 cm −1) of liquid S-phenyl thioacetate have been recorded. Molecular geometry, vibrational frequencies and the corresponding assignments were performed by density funtional theory (DFT) using the 6-311++G(d,p) basis set. Two conformers have been identified. One is syn form where the carbonyl group is on the same side of the phenyl ring and the other is the anti form. The energy difference between these two configurations is about 1.63 kcal mol −1 at B3LYP/6-311++G(d,p) level. By utilizing the more stable syn conformer, a complete assignment of the observed frequencies is given according to the total energy distribution of the vibrational modes. The general agreements between the observed and the calculated frequencies are shown.

Structural phase transition, elastic properties and electronic properties of chalcopyrite CuAlX2 (X=S, Se, Te)

Abstract The density functional theory calculations of structural phase transition, elastic constants and electronic structure of the CuAlX 2 (X = S, Se, Te) have been reported using the full potential linear muffin-tin orbital (FP-LMTO). In this approach, the local density approximation is used for the exchange-correlation potential. Results are given for lattice constant, bulk modulus, and its pressure derivative, band structure, density of states and pressure dependence of energy gaps. We note that these compounds are semiconductors with direct energy gaps. The valence-band maximum and conduction-band minimum are located at Γ . Our results are compared with previous theoretical calculations and experimental data. The elastic constants at equilibrium in BCT structure are also determined.

Forces and conductances in a single-molecule bipyridine junction

Inspired by recent measurements of forces and conductances of bipyridine nano-junctions, we have performed density functional theory calculations of structure and electron transport in a bipyridine molecule attached between gold electrodes for seven different contact geometries. The calculations show that both the bonding force and the conductance are sensitive to the surface structure, and that both properties are in good agreement with experiment for contact geometries characterized by intermediate coordination of the metal atoms corresponding to a stepped surface. The conductance is mediated by the lowest unoccupied molecular orbital, which can be illustrated by a quantitative comparison with a one-level model. Implications for the interpretation of the experimentally determined force and conductance distributions are discussed.

Infrared spectra of the CS2−, CS2+, and C2S4+ molecular ions in solid neon and argon

Laser ablation of transition metal targets with concurrent code position of CS2/Ne and CS2/Ar mixtures produces metal independent absorptions at 1206.8 and 1159.2 cm−1 in neon and 1200.5 and 1160.4 cm−1 in argon due to CS2+ and CS2−. Additional metal independent absorptions at 1385.2 cm−1 in neon and 1379.7 cm−1 in argon increase on annealing. Isotopic substitutions show that this vibration involves two equivalent CS2 subunits. Based on density functional theory calculations of structure and vibrational frequencies, the 1385.2 and 1379.7 cm−1 bands are assigned to the C2S4+ cation in solid neon and argon. Identical experiments using matrix samples doped with the electron trapping molecule CCl4 enhance the 1385.2 and 1379.7 cm−1 absorptions and further support the cation assignment. No evidence was found for the (CS2)2− anion in these experiments.

Infrared spectra of the C2O4+ cation and C2O4− anion isolated in solid neon

Laser ablation of transition metal targets with concurrent 4 K codeposition of CO2/Ne mixtures produces metal independent infrared absorptions at 1658.2 and 1421.6 cm−1 due to CO2− and CO2+. Additional metal independent absorptions at 2130.8 and 1274.4 cm−1, and at 1852.4, 1189.2, and 679.2 cm−1 increase on annealing to 8 K. Isotopic substitution shows that both band sets involve two equivalent CO2 subunits. Based on density functional theory calculations of structures and vibrational frequencies, the 2130.8 and 1274.4 cm−1 bands are assigned to the C2O4+ cation with C2h symmetry, and the 1852.4, 1189.2, and 679.2 cm−1 bands are assigned to the C2O4− anion with D2d symmetry. Evidence is also obtained for (CO2−)(CO2)x(x=1,2) anion complexes.