What is USPEX? USPEX is a method developed jointly by Artem R. Oganov, Andriy O. Lyakhov, Colin W. Glass and Qiang Zhu, and implemented in the same-name code written by Andriy O. Lyakhov, Colin W. Glass and Qiang Zhu. This method/code enables crystal structure prediction at arbitrary P-T conditions, given just the chemical composition of the material. Many previous attempts to solve crystal structure problem were plagued by low success rate and extreme computational costs that prevented full ab initio studies. USPEX avoids both of these problems. In fact, "uspekh" means "success" in Russian - which highlights a nearly 100% success rate that we find for our method. Today, USPEX is used by over 1100 researchers worldwide. The First Blind Test of Inorganic Crystal Structure Prediction (described in the book "Modern Methods of Crystal Structure Prediction, 2010) shows that USPEX outperforms other methods in terms of efficiency and reliability.
Figure. Test of USPEX: 40-atom cell of MgSiO3 post-perovskite. Left - structure search using local optimisation of random structures, Right - evolutionary search with USPEX. While random search did not produce the correct structure even after 120000 steps, USPEX found the stable structure in fewer than 1000 steps.
Features of the code.
(1) Prediction of the stable and metastable structures knowing only the chemical composition. Simultaneous searches for stable compositions and structures are also possible.
(2) Incorporation of partial structural information is possible - (a) constraining search to fixed experimental cell parameters, or fixed cell shape, or fixed cell volume, (b) starting structure search from known or hypothetical structures,
(c) assembling crystal structures from predefined molecules, including flexible molecules.
(3) efficient constraint techniques, which eliminate unphysical and redundant regions of the search space. Cell reduction technique (Oganov & Glass, 2008).
(5) initialization using fully random approach, or using space groups and cell splitting techniques (Lyakhov, Oganov, Valle, 2010)
(6) on-the-flight analysis of results - determination of space groups (and output in CIF-format), calculation of the hardness, order parameters, etc.
(7) prediction of the structure of nanoparticles and surface reconstructions
(8) restart facilities, enabling calculations to be continued from any point along the evolutionary trajectory
(9) powerful visualization and analysis techniques implemented in the STM4 code (by M.Valle), fully interfaced with USPEX.
(10) USPEX is interfaced with VASP, SIESTA, GULP, DMACRYS, CP2k, QuantumEspresso codes. Interfacing with other codes is easy.
(11) submission of jobs from local workstation to remote clusters and supercomputers is possible.
(12) job submission via grid is possible (grid part written by S. Tikhonov and S. Sobolev).
(13) options for structure prediction using the USPEX algorithm (default), random sampling, corrected particle swarm optimization, evolutionary metadynamics, minima hopping-like algorithm. Capabilities to predict phase transition mechanisms using evolutionary metadynamics, variable-cell NEB method, and TSP method.
(14) options to optimize physical properties other than the energy - e.g., hardness (Lyakhov & Oganov, 2011), density (Zhu et al., 2011), and various electronic properties.
(15) many new features are now in progress. to be described later...
Current limitations of USPEX. Because of the high success rate of the method, we have not seen many limitations in practice. It is efficient for systems with up to 100-200 atoms/cell. Difficulties for large systems are due to the increasing cost of ab initio calculations for increasing system sizes, and also due to the rapidly increasing number of energy minima. Our algorithm seems to be very effective in counteracting this effect and will make structure prediction for systems containing many hundreds of atoms affordable in near future.
How to collaborate on USPEX. We welcome collaborations with experimentalists finding new interesting phases and wishing to find their structure, and with industrial partners. We welcome all interested theoretical and computational scientists to join the development of USPEX. The best way to enquire about a possible collaboration is to e-mail Prof. A.R. Oganov.
Conditions for becoming a user of USPEX. The USPEX code is public domain, but as for any public code, there are certain conditions that users must sign to -
(i) the code is given to an individual researcher (not a group or institution), users are not allowed to distribute the code,
(ii) citations to the original USPEX publications must be present in all papers that used USPEX.
(iii) all new features that the users would like to implement will have to be sent to Prof. A.R. Oganov in order to be included in the common version of USPEX, maximally benefiting the user community. Users of USPEX are welcome to participate in the development of USPEX and will then be named as its coauthors, but will refrain from developing any competing codes.
References, where the method was exhaustively described:
1. Oganov A.R., Glass C.W. (2006). Crystal structure prediction using evolutionary algorithms: principles and applications. J. Chem. Phys.124, art. 244704 (pdf-file).
3. Oganov A.R., Glass C.W. (2008). Evolutionary crystal structure prediction as a tool in materials design. J. Phys.: Cond. Mattter 20, art. 064210 (pdf-file).
4. Oganov A.R., Ma Y., Lyakhov A.O., Valle M., Gatti C. (2010). Evolutionary crystal structure prediction as a method for the discovery of minerals and materials. Rev. Mineral. Geochem.71, 271-298 (pdf-file).
5. Lyakhov A.O., Oganov A.R., Valle M. (2010). How to predict very large and complex crystal structures. Comp. Phys. Comm.181, 1623-1632 (pdf-file).
6. Oganov A.R., Lyakhov A.O., Valle M. (2011). How evolutionary crystal structure prediction works - and why. Acc. Chem. Res.44, 227-237 (pdf-file).
Some of the (many) discoveries made using USPEX (send us your discoveries to include in this list!):
Prediction of the densest carbon materials
(Zhu Q., Oganov A.R., Salvado M., Pertierra P., Lyakhov A.O. (2011). Denser than diamond:ab initio search for superdense carbon allotropes. Phys. Rev. B83, 193410 (pdf-file)).
Prediction of the structure of graphane
(Wen X.D., Hand L., Labet V., Yang T., Hoffmann R., Ashcroft N.W., Oganov A.R., Lyakhov A.O. (2011). Graphane sheets and crystals under pressure. Proc. Natl. Acad. Sci. 108, 6833-6837 (pdf-file, Supporting Online Materials)).
High-pressure behavior of methane and its implications for the interiors of planet Neptune
(Gao G., Oganov A.R., Wang H., Li P., Ma Y., Cui T., Zou G. (2010). Dissociation of methane under high pressure. J. Chem. Phys. 133, 144508 (pdf-file)).
Study of exotic structures and superconductivity of calcium under pressure
(Oganov A.R., Ma Y.M., Xu Y., Errea I., Bergara A., Lyakhov A.O. (2010). Exotic behavior and crystal structures of calcium under pressure. Proc. Natl. Acad. Sci.107, 7646-7651 (pdf-file, Supplementary Material)).
Prediction of stable compounds LiH8, LiH6 and LiH2
(Zurek E., Hoffmann R., Ashcroft N.W., Oganov A.R., Lyakhov A.O. (2009). A little bit of lithium does a lot for hydrogen. Proc. Natl. Acad. Sci. 106, 17640-17643. (pdf-file, Supplementary Material))
Structure of superhard graphite:
(a - Li Q., Ma Y., Oganov A.R., Wang H., Wang H., Xu Y., Cui T., Mao H.-K., Zou G. (2009). Superhard monoclinic polymorph of carbon. Phys. Rev. Lett. 102, 175506. (pdf-file); b - Oganov A.R., Glass C.W. (2006). Crystal structure prediction using ab initio evolutionary techniques: principles and applications. J. Chem. Phys. 124, art. 244704 (pdf-file)).
Theoretical/experimental discovery of a transparent high-pressure phase of sodium:
(Ma Y., Eremets M.I., Oganov A.R., Xie Y., Trojan I., Medvedev S., Lyakhov A.O., Valle M., Prakapenka V. (2009). Transparent dense sodium. Nature 458, 182-185. (pdf-file, Supporting Online Material)).
Theoretical/experimental discovery of a new superhard and partially ionic phase of boron:
(a - Oganov A.R., Chen J., Gatti C., Ma Y.-Z., Ma Y.-M., Glass C.W., Liu Z., Yu T., Kurakevych O.O., Solozhenko V.L. (2009). Ionic high-pressure form of elemental boron. Nature 457, 863-867. (pdf-file, Supporting Online Material); b - Solozhenko V.L., Kurakevych O.O., Oganov A.R. (2008). On the hardness of a new boron phase, orthorhombic gamma-B28. J. Superhard Mater. 30, 428-429. (pdf-file)).