Alpha Quartz unit cell structure

Hello, everyone.
I creat sio2 unit cell by the follow:
lattice custom 5.4054 origin 0.25 0.25 0.25 &

a1 0.9095 0.0000 0.0000 &

a2 -0.4547 0.7876 0.0000 &

a3 0.0000 0.0000 1.0000 &

basis 0.4697 0.0000 0.0000 &

basis 0.0000 0.4697 0.6667 &

basis 0.5303 0.5303 0.3333 &

basis 0.4135 0.2669 0.1191 &

basis 0.2669 0.4135 0.5475 &

basis 0.7331 0.1466 0.7857 &

basis 0.5865 0.8534 0.2142 &

basis 0.8534 0.5865 0.4524 &

basis 0.1466 0.7331 0.8809
delete_atoms region sio21 mol yes compress yes
create_atoms 10 region sio21 basis 1 10 basis 2 10 basis 3 10 basis 4 11 & basis 5 11 basis 6 11 basis 7 11 basis 8 11 basis 9 11

but the number of si atom and o atom is 205 and 414 , the si:o isn’t 1:2. Is there anything wrong with the unit cell structure I built ?

I hope someone can give me some advice. Thank you very much!

zhang

please note that having a correct stoichiometry depends on how you cut through the lattice, i.e. define your region. this is often complicate, if you choose your region to coincide with the lattice points, that makes it ambiguous whether atoms located at the boundaries will be included or not. with more complex crystal structures this then can depend on floating point math rounding/truncation issues and for regions that are not the box, you also have then the issue that there is no test for overlaps due to periodic boundaries, so particles located exactly on the boundaries may be included on both sides. a good way to avoid this ambiguity and dependence of floating point accuracy is usually to shift the region boundaries away from the exact lattice points by a small margin.

axel.

Thank you very much for your generous answer, axel