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English N.J.,SFI Strategic Research Cluster in Solar Energy Conversion | English N.J.,University College Dublin
Chemical Physics Letters | Year: 2013

The dynamical properties of physically adsorbed water molecules on partially hydroxylated rutile-(1 10) surfaces have been studied via equilibrium Born-Oppenheimer molecular dynamics. It was found that water molecules were almost parallel to the surface. There is a coupling frequency of around 110 and 220 cm 1 between the translational modes of the water molecules and the upper layer of Ti and bridging oxygen atoms in the titania, respectively. Hydrogen bonds with the surface were observed to be stable, with stretch frequencies around 835-920 cm 1. Oxygen-vacancies were found to weaken hydrogen bonding between the water molecules' protons and the bridging oxygen atoms. © 2013 Elsevier B.V. All rights reserved. Source


Healy J.J.,Communications and Optoelectronic Research Center | Healy J.J.,SFI Strategic Research Cluster in Solar Energy Conversion | Healy J.J.,University College Dublin | Sheridan J.T.,Communications and Optoelectronic Research Center | Sheridan J.T.,University College Dublin
Optics Letters | Year: 2010

The linear canonical transform may be used to simulate the effect of paraxial optical systems on wave fields. Using a recent definition of the discrete linear canonical transform, phase space diagram analyses of the sampling requirements of the direct method of calculating the Fresnel and other linear canonical transforms are more favorable than previously thought. Thus the direct method of calculating these transforms may be used with fewer samples than previously reported simply by making use of an appropriate reconstruction filter on the samples output by the algorithm. © 2010 Optical Society of America. Source


Gorman P.D.,SFI Strategic Research Cluster in Solar Energy Conversion | English N.J.,SFI Strategic Research Cluster in Solar Energy Conversion | English N.J.,University College Dublin | MacElroy J.M.D.,SFI Strategic Research Cluster in Solar Energy Conversion
Physical Chemistry Chemical Physics | Year: 2011

Classical equilibrium molecular dynamics (MD) simulations have been performed to investigate the dynamical and energetic properties in hydrogen and mixed hydrogen-tetrahydrofuran sII hydrates at 30 and 200 K and 0.05 kbar, and also at intermediate temperatures, using SPC/E and TIP4P-2005 water models. The potential model is found to have a large impact on overall density, with the TIP4P-2005 systems being on average 1% more dense than their SPC/E counterparts, due to the greater guest-host interaction energy. For the lightly-filled mixed H2-THF system, in which there is single H2 occupation of the small cage (1s1l), we find that the largest contribution to the interaction energy of both types of guest is the van der Waals component with the surrounding water molecules in the constituent cavities. For the more densely-filled mixed H2-THF system, in which there is double H 2 occupation in the small cage (2s1l), we find that there is no dominant component (i.e., van der Waals or Coulombic) in the H2 interaction energy with the rest of the system, but for the THF molecules, the dominant contribution is again the van der Waals interaction with the surrounding cage-water molecules; again, the Coulombic component increases in importance with increasing temperature. The lightly-filled pure H2 hydrate (1s4l) system exhibits a similar pattern vis-à-vis the H 2 interaction energy as for the lightly-filled mixed H 2-THF system, and for the more densely-filled pure H2 system (2s4l), there is no dominant component of interaction energy, due to the multiple occupancy of the cavities. By consideration of Kubic harmonics, there is some evidence of preferential alignment of the THF molecules, particularly at 200 K; this was found to arise at higher temperatures due to transient hydrogen bonding of the oxygen atom in THF molecules with the surrounding cage-water molecules. © the Owner Societies 2011. Source


Wadnerkar N.,SFI Strategic Research Cluster in Solar Energy Conversion | English N.J.,SFI Strategic Research Cluster in Solar Energy Conversion | English N.J.,University College Dublin
Computational Materials Science | Year: 2013

Hybrid density functional theory (DFT) calculations were performed on the monoclinic clinobisvanite polymorph of bismuth vanadate (BiVO4), which has piqued experimental and theoretical interest in recent years due to its photo-active nature. In particular, the effect of using PBEsol and HSE06-PBEsol functional, as corrected for the solid state, is gauged upon the predictive performance for electronic structure, band gap and optical properties. It was found that HSEsol-type functionals offer better predictive performance than PBEsol with respect to experimental data, although PBEsol performs comparatively well for this system, despite its general well-known limitations. Nonetheless, appropriately-tuned HSE06-PBEsol hybrid functionals may provide an improved description of solids compared to PBE, PBEsol and HSE06. Although the 'standard' proportion of exact exchange for HSEsol (25%) seems to provide a relatively accurate band gap vis-à-vis experiment, a smaller proportion (circa 10%) achieves quantitative agreement in this respect; in addition, the prediction of the lattice constants and band gaps is improved substantially. For HSE06-PBEsol, analysis of the density of states analysis showed that the conduction-band minimum (CBM) and valence-band maximum (VBM) position shift upwards and downwards, respectively, thus widening the band gap, in better agreement with experimental results. It was found that clinobisvanite is an indirect (albeit closer to a direct) band-gap semiconductor, and the optical properties were investigated, including the effect of varying the proportion of exact exchange thereon. © 2013 Elsevier B.V. All rights reserved. Source


Healy J.J.,University College Dublin | Healy J.J.,SFI Strategic Research Cluster in Solar Energy Conversion | Sheridan J.T.,University College Dublin | Sheridan J.T.,SFI Strategic Research Cluster in Solar Energy Conversion
Journal of the Optical Society of America A: Optics and Image Science, and Vision | Year: 2010

The linear canonical transform provides a mathematical model of paraxial propagation though quadratic phase systems. We review the literature on numerical approximation of this transform, including discretization, sampling, and fast algorithms, and identify key results. We then propose a frequency-division fast linear canonical transform algorithm comparable to the Sande-Tukey fast Fourier transform. Results calculated with an implementation of this algorithm are presented and compared with the corresponding analytic functions. © 2010 Optical Society of America. Source

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