Hi All,
We’ve noticed a discrepancy between crystal system (primitive cubic Bravais lattice) and lattice parameters with γ 89.994°.
Thanks,
Sasha
Hi @sasha, welcome to the forum!
The data on the Materials Project is computational, and therefore subject to some amount of numerical noise. When we perform a symmetry analysis, we use a tolerance so that we’re robust against this noise. Therefore, crystals with angles that are very nearly almost 90º but not quite (e.g. 89.994º) will be rounded when we report the crystal system.
The reason for this is two-fold: (1) our simulation methods are not accurate enough to make a prediction accurate to 0.001º in lattice angles so some tolerance is sensible and (2) it is very difficult to measure lattice angles to this accuracy experimentally too.
I agree it’s a little confusing though that we report it in this way: perhaps we should just report the rounded angles, but we want to be transparent about the raw data that comes out of our calculations.
Hope this helps,
Matt
Thanks for the reply Matt!
Yes, I put the structure through the spglib (with symprec 1e-2) and it resulted in the cubic structure (same as “symmetrized” structure from the website).
Isn’t this concerning a bit that starting from (I believe) a cubic structure (http://rruff.geo.arizona.edu/AMS/minerals/Cuprite), during the workflow the symmetry is broken for the 3rd digit in the angle: round(89.99446098, 3) == 89.994.