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Danny E. P. Vanpoucke Dr.
Publication date
2013
fields
Physics
and research's language is
English
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The issues raised in the comment by T.A. Manz are addressed through the presentation of calculated atomic charges for NaF, NaCl, MgO, SrTiO$_3$ and La$_2$Ce$_2$O$_7$, using our previously presented method for calculating Hirshfeld-I charges in Solids [J. Comput. Chem.. doi: 10.1002/jcc.23088]. It is shown that the use of pseudo-valence charges is sufficient to retrieve the full all-electron Hirshfeld-I charges to good accuracy. Furthermore, we present timing results of different systems, containing up to over $200$ atoms, underlining the relatively low cost for large systems. A number of theoretical issues is formulated, pointing out mainly that care must be taken when deriving new atoms in molecules methods based on expectations for atomic charges.
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In this work, a method is described to extend the iterative Hirshfeld-I method, generally used for molecules, to periodic systems. The implementation makes use of precalculated pseudo-potential based charge density distributions, and it is shown that high quality results are obtained for both molecules and solids, such as ceria, diamond, and graphite. The use of such grids makes the implementation independent of the solid state or quantum chemical code used for studying the system. The extension described here allows for easy calculation of atomic charges and charge transfer in periodic and bulk systems.
Cazorla et al. [preceding comment] criticize our recent results on the high-PT phase diagram of CaF2 (Phys. Rev. B 95, 054118 (2017)). According to our analysis, Cazorla et al. have not converged their calculations with respect to simulation cell size, undermining the comments conclusions about both the high-T behaviour of the P-62m-CaF2 polymorph, and the use of the QHA in our work. As such, we take this opportunity to emphasise the importance of correctly converging molecular dynamics simulations to avoid finite-size errors. We compare our quasiharmonic phase diagram for CaF2 with currently available experimental data, and find it to be entirely consistent and in qualitative agreement with such data. Our prediction of a superionic phase transition in P-62m-CaF2 (made on the basis of the QHA) is shown to be accurate, and we argue that simple descriptors, such as phonon frequencies, can offer valuable insight and predictive power concerning superionic behaviour.
Magnetic coercivity is often viewed to be lower in alloys with negligible (or zero) values of the anisotropy constant. However, this explains little about the dramatic drop in coercivity in FeNi alloys at a non-zero anisotropy value. Here, we develop a theoretical and computational tool to investigate the fundamental interplay between material constants that govern coercivity in bulk magnetic alloys. The two distinguishing features of our coercivity tool are that: (a) we introduce a large localized disturbance, such as a spike-like magnetic domain, that provides a nucleation barrier for magnetization reversal; and (b) we account for magneto-elastic energy -- however small -- in addition to the anisotropy and magnetostatic energy terms. We apply this coercivity tool to show that the interactions between local instabilities and material constants, such as anisotropy and magnetostriction constants, are key factors that govern magnetic coercivity in bulk alloys. Using our model, we show that coercivity is minimum at the permalloy composition (Fe-21.5Ni-78.5) at which the alloys anisotropy constant is not zero. We systematically vary the values of the anisotropy and magnetostriction constants, around the permalloy composition, and identify new combinations of material constants at which coercivity is small. More broadly, our coercivity tool provides a theoretical framework to potentially discover novel magnetic materials with low coercivity.
An outstanding challenge of theoretical electronic structure is the description of van der Waals (vdW) interactions in molecules and solids. Renewed interest in resolving this is in part motivated by the technological promise of layered systems including graphite, transition metal dichalcogenides, and more recently, black phosphorus, in which the interlayer interaction is widely believed to be dominated by these types of forces. We report a series of quantum Monte Carlo (QMC) calculations for bulk black phosphorus and related few-layer phosphorene, which elucidate the nature of the forces that bind these systems and provide benchmark data for the energetics of these systems. We find a significant charge redistribution due to the interaction between electrons on adjacent layers. Comparison to density functional theory (DFT) calculations indicate not only wide variability even among different vdW corrected functionals, but the failure of these functionals to capture the trend of reorganization predicted by QMC. The delicate interplay of steric and dispersive forces between layers indicate that few-layer phosphorene presents an unexpected challenge for the development of vdW corrected DFT.
The above comment [E. I. Lashin, D. Dou, arXiv:1606.04738] claims that the paper Quantum Raychaudhuri Equation by S. Das, Phys. Rev. D89 (2014) 084068 [arXiv:1404.3093] has problematic points with regards to its derivation and implications. We show below that the above claim is incorrect, and that there are no problems with results of the above paper or its implications.
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