While modeling amorphous materials computationally, I have seen papers using periodic boundary conditions. But glasses/amorphous materials doesn't have any periodic nature. Is it some kind of approximation? Or do we have some other way to model aperiodic materials?
Yes, it is an approximation. It needs very big model which contains a lot of atoms whereas models for perfect crystals are quite small. It needs huge computer power, so, huge amount of money to use super-computer is required. Same things happen for modelings of impurities or defects.
There are models for glassy materials available. Check the works by this group http://blogs.uakron.edu/simmonsgroup/
Many attempts have been made to model amorphous materials. Periodic models are an approximation since amorphous materials are not periodic. One hopes that the model is large enough to capture the important structural features of the amorphous material. In reality one should sample many different periodic models to estimate the average features; this is especially important when examining amorphous surfaces (or catalysts on amorphous surfaces), which is more challenging to model than bulk amorphous materials, see e.g.,
Langmuir, 2014, 30 (18), pp 5133–5141
DOI: 10.1021/la500422p
Ind. Eng. Chem. Res., 2016, 55 (48), pp 12350–12357
DOI: 10.1021/acs.iecr.6b03558
One could also use cluster models instead of periodic slab models, but those have their own drawbacks and limitations. See e.g.,
Phys. Rev. B 71, 024208 (2005).
DOI: https://doi.org/10.1103/PhysRevB.71.024208
The Journal of Chemical Physics 138, 204105 (2013); DOI: http://dx.doi.org/10.1063/1.4807384
with my team we have modeled electrolyte and fluoride glass with the periodic boundary conditions, but these conditions are used just to reproduce the macroscopic system from an elementary simulation box, as in The MMC an the MD simulations for example. The periodic boundary conditions are not necessarily related to the structure. If you have any technical question on the use of the periodic boundary conditions do not hesitate.
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