We present a molecular dynamics simulation study of CH3NH3PbI3 based on forces calculated from density functional theory. The simulations were performed on model systems having 8 and 27 unit cells, and for a total simulation time of 40 ps in each case. Analysis of the finite size effects, in particular the mobility of the organic component, suggests that the smaller system is over-correlated through the long-range electrostatic interaction. In the larger system, this finite size artifact is relaxed, producing a more reliable description of the anisotropic rotational behavior of the methylammonium molecules. The thermal effects on the optical properties of the system were also analyzed. The HOMO-LUMO energy gap fluctuates around its central value with a standard deviation of approximately 0.1 eV. The projected density of states consistently place the Fermi level on the p orbitals of the I atoms and the lowest virtual state on the p orbitals of the Pb atoms throughout the whole simulation trajectory.

The structural, electronic, and vibrational properties of glassy Ge x Se 1−x are studied using density-functionalbased molecular dynamics. The focus is on four compositions (x = 10%,20%,25%,33%) spanning the rigidity transitions and representing typical compositions of flexible, intermediate, and stressed rigid systems. We investigate structural properties including structure factors, pair distribution functions, angular distributions, coordination numbers, and neighbor distributions and compare our results with experimental findings, when available. Most noticeable is the excellent agreement found in the reproduction of the structure in real and reciprocal space which allows tracking the effect of Ge composition on the structure. Ring statistics and ring correlations are examined and followed across the rigidity transition, and the details of typical small rings show a much more complex picture than established previously. A comparison is made with simple bond models and their validity is discussed. Topological constraint analysis is performed and shows that the onset of rigidity changes substantially the angular motion inside the Ge tetrahedra, which displays increased soft bending motions in the stressed rigid phase. We then investigate the vibrational properties via the vibrational density of states and the dielectric function (infrared absorption), and discuss them with respect to experimental findings. Finally, the electronic properties are computed and show an excellent agreement with respect to previous first-principles simulations and to experiments.

Oxothiomolybdenum wheels represent a new family of efficient electrocatalysts for the reduction of protons into hydrogen. The present study focuses on the complex [Mo8S8O8(OH)8(Ox)]2- (1) as a lithium salt (Ox2- = oxalate), which exhibits an electrocatalytic reduction wave of protons in the presence of perchloric acid at about −1.00 V versus SCE in DMF. Efficiency of this new electrocatalyst was evidenced by cyclic voltammetry and coulometry. DFT calculations were reported and bring about data of the protonated state of the catalyst, the first step of the postulated mechanism in the catalytic reaction. Other weak acids such as p-toluenesulfonic acid, trifluoroacetic acid, and acetic acid in DMF were tested and found to be efficient for the reduction of protons. The complex [Mo12S12O12(OH)12(Trim)]3- (2) behaves also as an electrocatalyst, functioning at a very low overpotential. Finally, the biomimetic behaviors of this new class of efficient electrocatalysts for hydrogen production mimic those of hydrogenases and compare favorably with their performances.