Authors: 
Danny E. P. Vanpoucke, Kurt Lejaeghere, Veronique Van Speybroeck, Michel Waroquier, and An
Ghysels 
Journal: 
J. Phys. Chem. C 119(41), 2375223766 (2015) 
doi: 
10.1021/acs.jpcc.5b06809 
IF(2015): 
4.509 
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<J.Phys.Chem.C> 

Graphical Abstract: Pulay stresses complicate the structure optimization of the breathing MIL47(V) MetalOrganic Framework. 
Modeling the flexibility of metal–organic frameworks (MOFs) requires the computation of mechanical properties from first principles, e.g., for screening of materials in a database, for gaining insight into structural transformations, and for force field development. However, this paper shows that computations with periodic density functional theory are challenged by the flexibility of these materials: guidelines from experience with standard solidstate calculations cannot be simply transferred to flexible porous frameworks. Our test case, the MIL47(V) material, has a largepore and a narrowpore shape. The effect of Pulay stress (cf. Pulay forces) leads to drastic errors for a simple structure optimization of the flexible MIL47(V) material. Pulay stress is an artificial stress that tends to lower the volume and is caused by the finite size of the plane wave basis set. We have investigated the importance of this Pulay stress, of symmetry breaking, and of kpoint sampling on (a) the structure optimization and (b) mechanical properties such as elastic constants and bulk modulus, of both the largepore and narrowpore structure of MIL47(V). We found that, in the structure optimization, Pulay effects should be avoided by using a fitting procedure, in which an equation of state E(V) (EOS) is fit to a series of energy versus volume points. Manual symmetry breaking could successfully lower the energy of MIL47(V) by distorting the vanadium–oxide distances in the vanadyl chains and by rotating the benzene linkers. For the mechanical properties, the curvature of the EOS curve was compared with the Reuss bulk modulus, derived from the elastic tensor in the harmonic approximation. Errors induced by anharmonicity, the eggbox effect, and Pulay effects propagate into the Reuss modulus. The strong coupling of the unit cell axes when the unit cell deforms expresses itself in numerical instability of the Reuss modulus. For a flexible material, it is therefore advisible to resort to the EOS fit procedure.
Oct 28
Comment on ‘Europium doping induced symmetry deviation and its impact on the second harmonic generation of doped ZnO nanowires’
2014, publications
Abstract
In Dhara et al. 2014 Nanotechnology 25 225202, the authors reported on the synthesis of Eudoped ZnO nanowires (NWs) and investigated the influence of Eu doping on the second harmonic generation (SHG). Maximum SHG was found to correlate strongly with the structural deformation attributed to Eu^{3+} doping. In this comment, we show the deformation of interest is due to the presence of Eu^{2+} dopants, based on both the experimental data presented by Dhara et al. and abinitio density functional theory calculations.
Tags: Comment, Doping, Experiment, Materials Science, Nanotechnology