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DFT tools e.g. vaspkit after post-processing elastic calculation results provide elastic properties, say, bulk modulus for bulk single crystal and bulk polycrystal. Bulk single crystal values are given in terms of minimum and maximum value of properties with anisotropy. Whereas bulk polycrystal values are given in Voigt (V), Reuss (R) and Hill (H) approximations separately. My question is- How does one come to know which crystal type values (single or polycrstal) to consider? In other words: what settings of DFT input parameters indicate single crystal or polycrystal nature of material being modelled? For polycrystal, do we need to take average of V, R and H values to get more correct elastic property?

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Voigt and Reuss bounds are considered as limits within which the polycrystalline elastic modulus can exist. Many other bounds have been proposed subsequently to bridge the gap between theoretical predictions and reality. In modeling, we often focus on single crystals or bicrystals using Density Functional Theory (DFT) because simulating larger grain sizes in polycrystals can be computationally expensive. With molecular dynamics, simulations on a few million atoms are feasible, but there are many complex phenomena that occur during loading of polycrystalline metals.

One notable challenge is the disparity in loading rates between experiments and molecular dynamics simulations, which can lead to different modulus values. So, what's the aim of modeling polycrystals? If the elastic modulus is the primary concern, then Voigt-Reuss-Hill relations can qualitatively predict the range within which the modulus should ideally exist.

For polycrystal, do we need to take average of V, R and H values to get more correct elastic property?

Compare with literature if exists

Useful paper: https://doi.org/10.1063/1.1709944

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  • $\begingroup$ Thank you @Pranav for you insights and sorry for coming late here. I have modeled BULK Ti3C2F2 MXene and computed its bulk modulus (B). I have got single crystal B values with min-max values and polycrystal B values with V,R and H bounds using VASPKIT. We need to pick only one value of bulk modulus for the formula for thermal expansion coefficient and Grüneisen parameter. To help decide on the one particular value to pick, there is no literature on BULK case. All I have is monolayer based literature. So, I need your kind suggestion on the one particular value to pick for further calculations. $\endgroup$
    – AbPhys
    Commented May 15 at 9:33
  • $\begingroup$ why not to calculate bounds for thermal coefficient and Gruneisen parameter as well using min and max? $\endgroup$ Commented May 15 at 11:22
  • $\begingroup$ That is also possible. But can't we take any single value of bulk modulus here like we get by fitting Birch-Murnaghan equation and using PhonoPy? $\endgroup$
    – AbPhys
    Commented May 15 at 11:39

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