Query Regarding Crystal Symmetry and Space Group Identification in QuantumATK

[UtpalLab123]

Dear Sir/Ma'am,

When I compute the crystal symmetry and space group number for my system (plane 2d system), I see an option in the QuantumATK-R-2020.09-SP1 version called "Crystal Symmetry Info." Under this, there are two parameters:
(a) Position, which defines the tolerance of the fractional coordinates when searching for the space group.
(b) Angle, which defines the tolerance of the angles when searching for the space group.
My question is: What values should I use for these parameters to accurately identify the crystal symmetry and space group number?

[Anders Blom]

If the structure is highly symmetric you can use a very low value, 1e-4 or so. In that case, you will also find that changing the tolerance doesn't change the identification.

If the system only has some approximate symmetry, you need to check a few different orders of magnitude and see if you get different results. That will tell you how close to a particular space group you are, and you need to use some judgement to say what is the most reasonable identification, within some uncertainty.

If you end up needing 0.1 or that range to get anything other than P1, probably the system has no real symmetries at all.