# What is the minimum supercell size for phonon spectrum calculation for this monolayer system? [closed]

Following the previous threads on this topic, I would like to know the minimum supercell size suitable for Phonon spectrum calculation using Phonopy (DFPT) for CrI$$_3$$ monolayer system given by this unit cell POSCAR:

 CrI3
1.00000000000000
7.2414772987377169   -0.0000334312315729   -0.0000273695524433
-3.6207634204735277    6.2714102717088256    0.0000236446352383
-0.0000690784193687    0.0000291507055479   18.1739997862014491
Cr   I
2     6
Direct
0.6666642998354509  0.3332969407635646  0.4999838454239838
0.3333680676615832  0.6666687700374014  0.5000267181089781
0.6449405535702791  0.9999734919584491  0.4102132677332540
0.3551208025009766  0.3550499098699756  0.4102112382804572
0.0000237365755498  0.6448837571827296  0.4102090416058436
0.3550928327367056 -0.0000708711957224  0.5897938401149568
0.6449229383444248  0.6449027058686945  0.5897995792122650
0.0000673177750284  0.3550765345149079  0.5898047365202591

0.00000000E+00  0.00000000E+00  0.00000000E+00
0.00000000E+00  0.00000000E+00  0.00000000E+00
0.00000000E+00  0.00000000E+00  0.00000000E+00
0.00000000E+00  0.00000000E+00  0.00000000E+00
0.00000000E+00  0.00000000E+00  0.00000000E+00
0.00000000E+00  0.00000000E+00  0.00000000E+00
0.00000000E+00  0.00000000E+00  0.00000000E+00
0.00000000E+00  0.00000000E+00  0.00000000E+00

• The supercell size (or equivalently $\mathbf{q}$-point grid size) is a convergence parameter, so the only way to answer this question is to perform tests with different supercell sizes. Oct 29, 2021 at 17:28
• You mean SCF tests ? Oct 29, 2021 at 22:02
• No, I mean doing calculations with an increasingly large supercell until the phonon dispersion/energy is converged. Oct 30, 2021 at 8:12
• On the phonopy sourcefourge forum, they suggest taking about 6 to 7 $\overset{\circ}{A}$ or a distance greater than the second nearest neighbour distance, whichever is larger, in each direction. Nov 2, 2021 at 11:16
• @ChiKou while you will be pinged for all comments on this post, you need to include @username for the user you are replying to in order for them to be notified.
– Tyberius
Nov 7, 2021 at 1:31