ElMDpy

This is a pure python implementation of the ElMD library, which excludes the numba dependency.

The Element Movers Distance (ElMD) is a similarity measure for chemical compositions. This distance between two compositions is calculated from the minimal amount of work taken to transform one distribution of elements to another along the modified Pettifor scale.

This repository provides the reference implementations as described in our paper "The Earth Movers Distance as a metric for the space of inorganic compositions".

If you wish to compute this metric between lots of compositions, the ElM2D high-performance library may be more useful and can be found at www.github.com/lrcfmd/ElM2D.

We recommend installation via pip

pip install ElMDpy

Usage

For simple usage initiate an object with its compositional formula

> from ElMDpy import ElMD
> x = ElMD("CaTiO3")

Calculate the distance to a second object with the elmd method.

> x.elmd("SrTiO3")
0.2

If the assignment plan (how each element in the source composition is mapped to the target composition) is required, this may be returned by setting the return_assignments flag in the elmd method.

> x.elmd("SrTiO3", return_assignments=True)
(0.2, array([0.2, 0. , 0. , 0. , 0.2, 0. , 0. , 0. , 0.6]))

If the mod_petti elemental scale is suitable and no assignment plan is required, a significantly faster EMD algorithm may be used by setting metric="fast"

> x = ElMD("CaTiO3", metric="fast")
> x.elmd("SrTiO3")
0.2

The compositional parser can handle user defined values of x when this is applicable.

latp_02 = ElMD("Li1+xAlxTi2-x(PO4)3", x=0.2) # Li1.2Al0.2Ti1.8(PO4)3
latp_03 = ElMD("Li1+xAlxTi2-x(PO4)3", x=0.3) # Li1.3Al0.3Ti1.7(PO4)3

Alternate chemical scales may be accessed via the "metric" argument, e.g.

> x = ElMD("CaTiO3", metric="atomic")
> x.elmd("SrTiO3")
3.6

The elmd() method is overloaded to take two strings, and may be imported directly. The choice of metric is specified with metric

from ElMD import elmd
> elmd("NaCl", "LiCl")
0.5
> elmd("NaCl", "LiCl", metric="magpie")
0.688539

The EMD function can also be called directly, with the input being two vectors of distributions and the associated distance matrix between them.

from ElMD import EMD
> EMD([0.5, 0.5], [0.5, 0.5], [[1, 90], [89, 0]])
0.5

Elemental Similarity

You may use either traditional discrete scales or machine learnt representations for each element. In this instance a vector has been generated for each element, and the distance between elements (not compositions!) is the Euclidean distance.

Due to the disparity in magnitudes of some of these values, a select few have additionally been scaled.

Linear:

• mendeleev
• petti
• atomic
• mod_petti

Chemically Derived:

• oliynyk
• oliynyk_sc
• jarvis
• jarvis_sc
• magpie
• magpie_sc

Machine Learnt:

• cgcnn
• elemnet
• mat2vec
• matscholar
• megnet16

Random Numbers:

• random_200

The Euclidean distance between these vectors is taken as the measure of elemental similarity.

> x = ElMD("NaCl", metric="magpie")
> x.elmd("LiCl")
46.697806

> x = ElMD("NaCl", metric="magpie_sc")
> x.elmd("LiCl")
0.688539

The feature dictionary can be accessed through the periodic_tab attribute:

> featurizingDict = ElMD(metric="magpie).periodic_tab
> featurizingDict["Na"]
[2.0, 22.98976928, 370.87, 1.0, 3.0, 166.0, 0.93, 1.0, 0.0, 0.0, 0.0, 1.0, 1.0, 0.0, 0.0, 0.0, 1.0, 29.2433333333, 0.0, 0.0, 229.0]

Featurizing

Whilst not the initial purpose, a compositional based feature vector may be generated from ElMD objects should you require it. This is a mean pooling of the weighted composition feature matrix.

Note that this vector representation is not used at any point during the ElMD distance calculation and is provided solely for convenience.

We construct this by taking the dot product of the ratios of each element with the features of these elements. Pass the argument feature_pooling="mean" to divide by the total number of elements in the compound.

feature_vector = np.dot(ratio_vector, element_feature_matrix)

This is accessed through the feature_vector attribute.

# For single element compositions, equivalent to x.periodic_tab["Cl"]
> x = ElMD("Cl", metric="magpie")
> x.feature_vector
array([ 94.    ,  35.453 , 171.6   ,  17.    ,   3.    , 102.    ,
         3.16  ,   2.    ,   5.    ,   0.    ,   0.    ,   7.    ,
         0.    ,   1.    ,   0.    ,   0.    ,   1.    ,  24.4975,
         2.493 ,   0.    ,  64.    ])

# Aggregate vector by each elements contribution
> x = ElMD("NaCl", metric="magpie").feature_vector
array([ 48.        ,  29.22138464, 271.235     ,   9.        ,
         3.        , 134.        ,   2.045     ,   1.5       ,
         2.5       ,   0.        ,   0.        ,   4.        ,
         0.5       ,   0.5       ,   0.        ,   0.        ,
         1.        ,  26.87041667,   1.2465    ,   0.        ,
       146.5       ])

A feature vector of length 8076 can be generated by concatenating the weighted mean, min, max, range, and standard deviation across each available elemental feature across all featurizing dictionaries for each element in the composition by calling the full_feature_vector() method.

> x = ElMD("NaCl").full_feature_vector()

Documentation

Complete documentation may be found at www.elmd.io/api

Citing

If you would like to cite this code in your work, please use the Chemistry of Materials reference

@article{doi:10.1021/acs.chemmater.0c03381,
    author = {Hargreaves, Cameron J. and Dyer, Matthew S. and Gaultois, Michael W. and Kurlin, Vitaliy A. and Rosseinsky, Matthew J.},
    title = {The Earth Mover’s Distance as a Metric for the Space of Inorganic Compositions},
    journal = {Chemistry of Materials},
    volume = {32},
    number = {24},
    pages = {10610-10620},
    year = {2020},
    doi = {10.1021/acs.chemmater.0c03381},
    URL = { 
        https://doi.org/10.1021/acs.chemmater.0c03381
    },
    eprint = { 
        https://doi.org/10.1021/acs.chemmater.0c03381
    }
}

Issues

Please feel free to post any questions or comments as issues on this GitHub page.