Request for PIEFACE site featurizer

[mkhorton]

J. Cumby and J. P. Attfield, Ellipsoidal Analysis of Coordination Polyhedra, Nature Communications 14235 (2017), see https://pieface.readthedocs.io/en/latest/ and https://github.com/jcumby/PIEFACE

Haven't investigated the easiest way to call it; suspect the most sensible option would be to call e.g. CrystalNN to get neighbor points and then their Ellipsoid class directly.

Feel free to close if this doesn't seem useful -- I don't have strong opinions, but seems like a nice supplement to other local env methods.

[ardunn]

@mkhorton were you imagining the ellipsoid parameters would be the output of this featurizer?

[mkhorton]

Yes, essentially the displacement from centre (of the centre atom from the ellipsoid centre), and the shape parameter (a measure of the ellipsoid distortion from ideal). Haven't looked into it much though; came across it when helping someone else trying to characterize their Jahn-Teller distortions.

[ardunn]

Superseded by #635