We investigate by first-principles molecular dynamics the structural properties of liquid GeSe 4 , i.e., Ge x Se 1Ϫx at xϭ0.2. This composition is very close to the so-called stiffness threshold composition, at which dramatic changes in a series of experimental properties occur. The calculated total neutron structure factor is in very good agreement with experiment. The results show that liquid GeSe 4 is a good prototype of a chemically ordered network. It consists of GeSe 4 tetrahedra that are connected by either shared Se atoms or Se chains.

A molecular-level understanding of why the addition of lithium salts to Organic Ionic Plastic Crystals (OIPCs) produces excellent ionic conductivity is described for the first time. These materials are promising electrolytes for safe, robust lithium batteries, and have been experimentally characterised in some detail. Here, molecular dynamics simulations demonstrate the effects of lithium ion doping on both the structure and dynamics of an OIPC matrix (tetramethylammonium dicyanamide [TMA][DCA]) and illustrate a molecular-level transport model: in the plastic crystal phase lithium ions can form clusters with [DCA](-), and this clustering then in turn creates free volume or defect paths in the remainder of the lattice, which enhances ion conduction.

We introduce a simple semiempirical anharmonic Kirkwood-Keating potential to model A x B 1−x C-type semiconductors. The potential consists of the Morse strain energy and Coulomb interaction terms. The optical constants of pure components, AB and BC, were employed to fit the potential parameters such as bond-stretching and -bending force constants, dimensionless anharmonicity parameter, and charges. We applied the potential to finite temperature molecular-dynamics simulations on Al x Ga 1−x As for which there is no lattice mismatch. The results were compared with experimental data and those of harmonic Kirkwood-Keating model and of equation-of-motion molecular-dynamics technique. Since the Morse strain potential effectively describes finite temperature damping, we have been able to numerically reproduce experimentally obtained optical properties such as dielectric functions and reflectance. This potential model can be readily generalized for strained alloys.

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We model lattice-mismatched group III-V semiconductor InxGa1-xAs alloys with the three-parameter anharmonic Kirkwood-Keating potential, which includes realistic distortion effect by introducing anharmonicity. Although the potential parameters were determined based on optical properties of the binary parent alloys InAs and GaAs, simulated dielectric functions, reflectance, and Raman spectra of alloys agree excellently with experimental data for any arbitrary atomic composition. For a wide range of atomic composition, InAs-and GaAs-bond retain their respective properties of binary parent crystals despite lattice and charge mismatch. It implies that use of the anharmonic Kirkwood-Keating potential may provide an optimal model system to investigate diverse and unique optical properties of quantum dot heterostructures by circumventing potential parameter searches for particular local structures.