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The structure and vibrational properties of high- and low-density amorphous (HDA and LDA, respectively) ices have been determined using reverse Monte Carlo, molecular dynamics, and lattice dynamics simulations. This combined approach leads to a more accurate and detailed structural description of HDA and LDA ices when compared to experiment than was previously possible. The water molecules in these ices form well connected hydrogen-bond networks that exhibit modes of vibration that extend throughout the solid and can involve up to 70% of all molecules. However, the networks display significant differences in their dynamical behavior. In HDA, the extended low-frequency vibrational modes occur in dense parallel two dimensional layers of water that are approximately 10 nm thick. In contrast, the extended modes in LDA resemble a holey structure that encapsulates many small pockets of nonparticipating water molecules. [ABSTRACT FROM AUTHOR]

The quantum transport of a gated polythiophene nanodevice is analyzed using density functional theory and nonequilibrium Green’s function approach. For this typical molecular field effect transistor, we prove the existence of two main features of electronic components, i.e., negative differential resistance and good switching. Ab initio based explanations of these features are provided by distinguishing fixed and shifting conducting states, which are shown to arise from the interface and functional molecule, respectively. The results show that proper functional molecules can be used in conjunction with metallic electrodes to achieve basic electronics functionality at molecular length scales. [ABSTRACT FROM AUTHOR]

Dynamic properties of β-hydroquinone (Qβ) clathrate with various guest molecules have been calculated by the method of lattice dynamics within the atom–atom potential approximation; van der Waals interactions, H-bond interactions, and electrostatic interactions for charges localized on the atoms are included. External vibration spectra have been calculated for the empty framework of Qβ and with different guest molecules (HCN, CO2, SO2, C2N2, C2H6, CH3OH, O2CO). Vibrational frequencies of the guest molecules in cavities are determined. The phonon density of states for these compounds has been calculated. The influence of the encaged guest molecules on the vibrational spectrum of the host lattice has been investigated. The most visible changes are observed upon the insertion of guest molecules with large dipole moment or those positioned asymmetrically inside the Qβ clathrate cages. Peculiarities of different interactions have been revealed. Analysis of the calculated and experimental results have been carried out. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The electronic and magnetic properties of double-impurities-doped TiO2 (rutile) are explored using first-principles calculations within the generalized gradient approximation to examine their potential use as spintronic system. Calculations are performed on all possible sets of double impurities (M1 and M2) from M1=Cr, Mn, Fe, Co, and Ni, and M2=Mo, W, and Re. The results show the overlapping of the highest occupied impurity states with the bottom of the conduction band of the host system and the half-metallic nature in Fe- and W-doped TiO2, making them suitable as spintronic systems. These impurities preferentially substitute adjacent Ti sites and form a stable magnetic complex with oxygen atoms. [ABSTRACT FROM AUTHOR]

The geometry and electronic structures of metal 5, 15-di-(4-thiophenyl)-porphyrin (MDTP) have been investigated using first-principles calculations. Several metal atoms, including Cr, Mn, Fe, Co, Ni, Cu and Zn, have been selected. It has been found that the dihedral angle between porphyrin and thiophenyl planes is close to the perpendicular, which means that the π conjugation in whole MDTP molecules is broken. The most stable spin configurations of Cr, Mn, Fe, Co and NiDTPs are S =2, 5/2, 1, 1/2 and 0, respectively. Analysis of metal 3d-orbital splitting in Zn and NiDTP have shown that in the case of NiDTP, the out-of-plane interaction between metal 3d-orbitals and π orbital of porphyrin is larger than that in the case of ZnDTP. The results suggest that the Ni metal will enhance the conductance of DTP because transport properties in molecular systems have strong relations to the molecular π orbital. [ABSTRACT FROM AUTHOR]