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Create structure-similarity.md #36

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@oxana-a oxana-a commented Jan 23, 2019

@tschaume ready to merge
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@oxana-a added a few comments that need to be addressed before I can merge

We use a novel method called CrystallNN to find near(est) neighbors in periodic structures. While the method will be introduced shortly[^3], it is already available through the python package [pymatgen](https://github.com/materialsproject/pymatgen).

## Site Fingerprints
The second step of the structure similarity calculation is the computation of a crystal site fingerprint, $v^{site}$, for each site in the two structures. The fingerprint is a 61-dimensional vector in which each element carries information about the local coordination environment computed with the *site* module of the python package [matminer](https://github.com/hackingmaterials/matminer). For example, the first two elements "wt $\ce{CN1}$" and "single bond $\ce{CN1}$" provide estimates of the likelihood (or weight) of how much the given site should be considered 1-fold coordinated (i.e., *w*$|_{CN=1}$). The third element "wt $\ce{CN2}$" provides a 2-fold coordination likelihood, whereas the fourth element "L-shaped $\ce{CN2}$" holds the resemblance similarity to an L-shaped coordination geometry (also called local structure order parameter) given that we find a coordination configuration with 2 atoms ($q_{L}|_{CN=2}$). The local structure order parameters can assume values between 0, meaning that the observed local environment has no resemblance with the target motif to which it is compared, and 1, which stands for perfect motif match. The remaining elements are: "water-like $\ce{CN2}$," "bent 120 degrees $\ce{CN2}$," "bent 150 degrees $\ce{CN2}$," "linear $\ce{CN2}$," "wt $\ce{CN3}$," "trigonal planar $\ce{CN3}$," "trigonal non-coplanar $\ce{CN3}$," "T-shaped $\ce{CN3}$," "wt $\ce{CN4}$," "square co-planar $\ce{CN4}$," "tetrahedral $\ce{CN4}$," "rectangular see-saw-like $\ce{CN4}$," "see-saw-like $\ce{CN4}$," "trigonal pyramidal $\ce{CN4}$," "wt $\ce{CN5}$," "pentagonal planar $\ce{CN5}$," "square pyramidal $\ce{CN5}$," "trigonal bipyramidal $\ce{CN5}$," "wt $\ce{CN6}$," "hexagonal planar $\ce{CN6}$," "octahedral $\ce{CN6}$," "pentagonal pyramidal $\ce{CN6}$," "wt $\ce{CN7}$," "hexagonal pyramidal $\ce{CN7}$," "pentagonal bipyramidal $\ce{CN7}$," "wt $\ce{CN8}$," "body-centered cubic $\ce{CN8}$," "hexagonal bipyramidal $\ce{CN8}$," "wt $\ce{CN9}$," "q2 $\ce{CN9}$," "q4 $\ce{CN9}$," "q6 $\ce{CN9}$," "wt $\ce{CN10}$," "q2 $\ce{CN10}$," "q4 $\ce{CN10}$," "q6 $\ce{CN10}$," "wt $\ce{CN11}$," "q2 $\ce{CN11}$," "q4 $\ce{CN11}$," "q6 $\ce{CN11}$," "wt $\ce{CN12}$," "cuboctahedral $\ce{CN12}$," "q2 $\ce{CN12}$," "q4 $\ce{CN12}$," "q6 $\ce{CN12}$," "wt $\ce{CN13}$," "wt $\ce{CN14}$," "wt $\ce{CN15}$," "wt $\ce{CN16}$," "wt $\ce{CN17}$," "wt $\ce{CN18}$," "wt $\ce{CN19}$," "wt $\ce{CN20}$," "wt $\ce{CN21}$," "wt $\ce{CN22}$," "wt $\ce{CN23}$," and "wt $\ce{CN24}$." Note that $q_{n}$ refers to Steinhardt bond orientational order parameter of order *n*. The resulting site fingerprint is thus defined as:
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The math in this paragraph doesn't come out right at all when rendered.


## Site Fingerprints
The second step of the structure similarity calculation is the computation of a crystal site fingerprint, $v^{site}$, for each site in the two structures. The fingerprint is a 61-dimensional vector in which each element carries information about the local coordination environment computed with the *site* module of the python package [matminer](https://github.com/hackingmaterials/matminer). For example, the first two elements "wt $\ce{CN1}$" and "single bond $\ce{CN1}$" provide estimates of the likelihood (or weight) of how much the given site should be considered 1-fold coordinated (i.e., *w*$|_{CN=1}$). The third element "wt $\ce{CN2}$" provides a 2-fold coordination likelihood, whereas the fourth element "L-shaped $\ce{CN2}$" holds the resemblance similarity to an L-shaped coordination geometry (also called local structure order parameter) given that we find a coordination configuration with 2 atoms ($q_{L}|_{CN=2}$). The local structure order parameters can assume values between 0, meaning that the observed local environment has no resemblance with the target motif to which it is compared, and 1, which stands for perfect motif match. The remaining elements are: "water-like $\ce{CN2}$," "bent 120 degrees $\ce{CN2}$," "bent 150 degrees $\ce{CN2}$," "linear $\ce{CN2}$," "wt $\ce{CN3}$," "trigonal planar $\ce{CN3}$," "trigonal non-coplanar $\ce{CN3}$," "T-shaped $\ce{CN3}$," "wt $\ce{CN4}$," "square co-planar $\ce{CN4}$," "tetrahedral $\ce{CN4}$," "rectangular see-saw-like $\ce{CN4}$," "see-saw-like $\ce{CN4}$," "trigonal pyramidal $\ce{CN4}$," "wt $\ce{CN5}$," "pentagonal planar $\ce{CN5}$," "square pyramidal $\ce{CN5}$," "trigonal bipyramidal $\ce{CN5}$," "wt $\ce{CN6}$," "hexagonal planar $\ce{CN6}$," "octahedral $\ce{CN6}$," "pentagonal pyramidal $\ce{CN6}$," "wt $\ce{CN7}$," "hexagonal pyramidal $\ce{CN7}$," "pentagonal bipyramidal $\ce{CN7}$," "wt $\ce{CN8}$," "body-centered cubic $\ce{CN8}$," "hexagonal bipyramidal $\ce{CN8}$," "wt $\ce{CN9}$," "q2 $\ce{CN9}$," "q4 $\ce{CN9}$," "q6 $\ce{CN9}$," "wt $\ce{CN10}$," "q2 $\ce{CN10}$," "q4 $\ce{CN10}$," "q6 $\ce{CN10}$," "wt $\ce{CN11}$," "q2 $\ce{CN11}$," "q4 $\ce{CN11}$," "q6 $\ce{CN11}$," "wt $\ce{CN12}$," "cuboctahedral $\ce{CN12}$," "q2 $\ce{CN12}$," "q4 $\ce{CN12}$," "q6 $\ce{CN12}$," "wt $\ce{CN13}$," "wt $\ce{CN14}$," "wt $\ce{CN15}$," "wt $\ce{CN16}$," "wt $\ce{CN17}$," "wt $\ce{CN18}$," "wt $\ce{CN19}$," "wt $\ce{CN20}$," "wt $\ce{CN21}$," "wt $\ce{CN22}$," "wt $\ce{CN23}$," and "wt $\ce{CN24}$." Note that $q_{n}$ refers to Steinhardt bond orientational order parameter of order *n*. The resulting site fingerprint is thus defined as:
$$
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This math doesn't get rendered either

## Structure Fingerprints
The fingerprints from sites in a given structure are subsequently statistically processed to yield the minimum, maximum, mean, and standard deviation of each coordination information element. The resultant ordered vector defines a structure fingerprint, $v^{struct}$:

$$
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also not rendered

## Structure Distance/Dissimilarity
Finally, structure similarity is determined by the distance, *d*, between two structure fingerprints $v_{i}^{struct}$ and $v_{j}^{struct}$:

$$
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not rendered


Below is a python code snippet that allows you to quickly reproduce above results. You will need to install [pymatgen](https://github.com/materialsproject/pymatgen) and [matminer](https://github.com/hackingmaterials/matminer) for this to work. Both are easily accessible via the [Python Package Index](https://pypi.python.org/pypi).

```python
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maybe long lines can be shortened to avoid having to scroll horizontally?

[^3]: H. Pan, J. Dagdelen, N. E. R. Zimmermann, A. Jain, in preparation (2018)

## Authors
Nils Zimmermann, Donny Winston, Handong Ling, Oxana Andriuc
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make authors an unnumbered list

@shyamd shyamd merged commit 78b766b into materialsproject:master Jun 16, 2021
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