The high-pressure alpha/beta phase transition in lead sulphide (PbS): X-ray powder diffraction and quantum mechanical calculations

The high-pressure behaviour of PbS was investigated by angular dispersive X-ray powder diffraction up to pressures of 6.8 GPa. Experiments were accompanied by first principles calculations at the density functional theory level. By combining both methods reliable data for the elastic properties of rock-salt type alpha- and high-pressure beta-PbS could be obtained. beta-PbS could be determined to crystallise in the CrB-type (B33), with space group Cmcm. The reversible ferro-elastic alpha/beta transition is of first order. It is accompanied by a large volume discontinuity of about 5% and a coexistence region of the two phases. A gliding mechanism of {001} bilayers along one of the cubic -directions governs the phase transition which can be described in terms of group/subgroup relationships via a common subgroup, despite its reconstructive character. The quadrupling of the primitive unit cell indicates a wave vector (0,0,pi/a) on the Delta-line of the Brillouin zone.Comment: 7 pages, 5 figures, 3 tables submitted to EP

Enhanced sampling in molecular dynamics using metadynamics, replica-exchange, and temperature-acceleration

We review a selection of methods for performing enhanced sampling in molecular dynamics simulations. We consider methods based on collective variable biasing and on tempering, and offer both historical and contemporary perspectives. In collective-variable biasing, we first discuss methods stemming from thermodynamic integration that use mean force biasing, including the adaptive biasing force algorithm and temperature acceleration. We then turn to methods that use bias potentials, including umbrella sampling and metadynamics. We next consider parallel tempering and replica-exchange methods. We conclude with a brief presentation of some combination methods. \ua9 2013 by the author; licensee MDPI, Basel, Switzerland

Ab initio vypocty fononovych spekter

Available from STL Prague, CZ / NTK - National Technical LibrarySIGLECZCzech Republi

Theoretical investigation of bonding in diaspore

The bulk modulus of diaspore, α-A1OOH, has been obtained from density functional theory based calculations. The value obtained, β = 148 GPa, is consistent with that previously obtained from elastic constant measurements, but in strong disagreement with values derived from high pressure x-ray diffraction experiments. A Mulliken bond population analysis of the electronic structure implies that the A1-O bonds are significantly covalent, in contrast to findings based on an earlier x-ray diffraction study. On compression, the main change is the increase in the hydrogen-bond strength

Calculation of the elastic constants of the Al2SiO5 polymorphs andalusite, sillimanite and kyanite

Based on quantum mechanical calculations we predict the elastic constants of kyanite at 0 K. The reliability of the prediction has been evaluated by computing the elastic constants of andalusite and of sillimanite and comparing them to experimentally determined values. The computed bulk module of andalusite (145 GPa) and of sillimanite (159 GPa) are consistent with experimental values. Only two of the computed elastic constants, c13 of andalusite and c23 of sillimanite differ from the experimental values by more than 11%. As the parameter-flee model is transferable, the predictions for the bulk modulus, B = 178 GPa, and the elastic constants of kyanite are believed to be equally reliable. In contrast to the promising results of our quantum mechanical calculations, the agreement with experimental values is poor for elastic properties derived from a transferable empirical core-shell model