arXiv subjects

Qiang Zhu

Publications and source records attributed to Qiang Zhu.

130 records · Page 8Linked to original sources

arXiv2012

Evolutionary Metadynamics: a Novel Method to Predict Crystal Structures

A novel method for crystal structure prediction, based on metadynamics and evolutionary algorithms, is presented here. This technique can be used to produce efficiently both the ground state and metastable states easily reachable from a reasonable initial structure. We use the cell shape as collective variable and evolutionary variation operators developed in the context of the USPEX method [Oganov, Glass, \textit{J. Chem. Phys.}, 2006, \textbf{124}, 244704; Lyakhov \textit{et al., Comp. Phys. Comm.}, 2010, \textbf{181}, 1623; Oganov \textit{et al., Acc. Chem. Res.}, 2011, \textbf{44}, 227] to equilibrate the system as a function of the collective variables. We illustrate how this approach helps one to find stable and metastable states for Al$_2$SiO$_5$, SiO$_2$, MgSiO$_3$, and carbon. Apart from predicting crystal structures, the new method can also provide insight into mechanisms of phase transitions.

cond-mat.mtrl-sci↗

arXiv2012

Systematic search for low-enthalpy sp3 carbon using evolutionary metadynamics

We present a systematic search for low-energy metastable superhard carbon allotropes by using the recently developed evolutionary metadynamics technique. It is known that cold compression of graphite produces an allotrope at 15-20 GPa. Here we look for all low-enthalpy structures accessible from graphite. Starting from 2H- or 3R-graphite and applying the pressure of 20 GPa, a large variety of intermediate $sp^3$ carbon allotropes were observed in evolutionary metadynamics simulation. Our calculation not only found all the previous proposed candidates for `superhard graphite', but also predicted two allotropes (\emph{X}-carbon and \emph{Y}-carbon) showing novel 5+7 and 4+8 topologies. These superhard carbon allotropes can be classified into five families based on 6 (diamond/lonsdaleite), 5+7 (\emph{M/W}-carbon), 5+7 (\emph{X}-carbon), 4+8 (bct C$_4$), and 4+8 (\emph{Y}-carbon) topologies. This study shows evolutionary metadynamics is a powerful approach both to find the global minima and systematically search for low-energy metastable phases reachable from given starting materials.

cond-mat.mtrl-sci↗

arXiv2012

Constrained evolutionary algorithm for structure prediction of molecular crystals: methodology and applications

Evolutionary crystal structure prediction proved to be a powerful approach for studying a wide range of materials. Here, we present a specifically designed algorithm for the prediction of the structure of complex crystals consisting of well-defined molecular units. The main feature of this new approach is that each unit is treated as a whole body, which drastically reduces the search space and improves the efficiency, but necessitates the introduction of new variation operators described here. To increase diversity of the population of structures, the initial population and part($\scriptsize{\sim}$20%) of the new generations are generated using space group symmetry combined with random cell parameters and random positions and orientations of molecular units. We illustrate the efficiency and reliability of this approach by number of tests (ice, ammonia, carbon dioxide, methane, benzene, glycine and butane-1,4-diammonium dibromide). This approach easily predicts the crystal structure of methane \emph{A} containing 21 methane molecules (105 atoms) per unit cell. We demonstrate that this new approach has also a high potential for the study of complex inorganic crystals on the examples of a complex hydrogen storage material Mg(BH$_4$)$_2$ and elemental boron.

cond-mat.mtrl-sci↗

arXiv2011

Extra connectivity measures of 3-ary n-cubes

The h-extra connectivity is an important parameter to measure the reliability and fault tolerance ability of large interconnection networks. The k-ary n-cube is an important interconnection network of parallel computing systems. The 1-restricted connectivity of k-ary n-cubes has been obtained by Chen et al. for k > 3 in [Y.-C. Chen, J. J. M. Tan, Restricted connectivity for three families of interconnection networks, Applied Mathematics and Computation 188 (2) (2007)1848--1855]. Nevertheless, the h-extra connectivity of 3-ary n-cubes has not been obtained yet. In this paper we prove that the 1-extra connectivity of a 3-ary n-cube is 4n-3 for n> 1 and the 2-extra connectivity of 3-ary n-cube is 6n-7 for n> 2.

cs.DM↗