# ChemEnv : a fast and robust tool to automatically identify coordination environments¶

The tutorial was written by Janine George (Contact: janine.george(at)uclouvain.be, Université catholique de Louvain, https://jageo.github.io/) in collaboration with Geoffroy Hautier (Université catholique de Louvain) and David Waroquiers (Université catholique de Louvain). The tutorial is based on a sample file by David Waroquiers.

Written using:

• pymatgen==2019.2.24

This tutorial will show you how to use ChemEnv to analyze chemical coordination environments and to identify model coordination environments that are closest to the coordination environment at hand. The latter is decided by the calculation of continous symmetry measures (CSM).

The analysis of the coordination environments proceeds as follows:

1. Search for neighbors by a modified Voronoi analysis on a grid of distance and angular parameters
2. Calculation of corresponding continous symmetry measures (CSMs) with model environments for all distance and angular parameters
3. Refinement of the results with different strategies that will be explained below (e.g., by using certain distance and angular parameters)

More information can be found in: D. Waroquiers et al., Chem Mater., 2017, 29, 8346

## First steps¶

3. Setup the connection to the materials project with: pmg config --add PMG_MAPI_KEY <USER_API_KEY>

If you only want to analyse very few structures, you can also use the get_environment script. Just type get_environment and follow the instructions. It will use the SimplestChemenvStrategy with default settings (i.e. distance_cutoff=1.4, angle_cutoff=0.3, Only anion-cation bonds and continuous_symmetry_measure_cutoff=10.0 ). With this script, you can also visualize the coordination environments. To do so, please install vtk (e.g. via pip3 install vtk). Moreover, you can analyze the dependency of the coordination environments on the distance and angular parameters. Just use the 'grid' setting after the question "See list of environments determined for each (unequivalent) site ?" and choose a site. Now, a tutorial to write your own scripts to analyze the coordination environments will follow.

## Import the relevant modules¶

Let us start by importing the relevant modules.

## Load a structure from the materials project¶

Next, we will load the crystal structure of our interest. We will start with a very simple example here: α-quartz which is depicted below.

Here, the structure of alpha-SiO2 (mp-7000) is depicted with possible coordination environments for Si (blue).

The graphic is created with VESTA: K. Momma and F. Izumi, "VESTA 3 for three-dimensional visualization of crystal, volumetric and morphology data," J. Appl. Cryst., 2011, 44, 1272.

## Setup of the local geometry finder (Voronoi analysis and csm on a grid follow)¶

Here, the Voronoi analysis will be set up. Have a look at the the code for more information. Moreover, a logging is introduced. This is especially important if one runs very large calculations (large distances and small angle parameters) and one wants to access the status of the calculation.

## Get the StructureEnvironments (=Voronoi analysis plus CSM calculation on a grid of distance and angular parameters)¶

In the first step, sets of neighbors for different distance and solid angle parameters via Voronoi analysis and corresponding continous symmetry measures (CSMs) are computed.

Have a close look at the documentation of the code for more information on parameters that you can define. For example, we will use the maximum_distance_factor to save some computing time. Multiplied with the distance to the closest neighbor it results in the maximum distance that is considered in the Voronoi approach. Similarily, one can define another parameter - the minimum_angle_factor. This factor is multiplied with the maximum solid angle to the neighbors to arrive at the minimum solid angle that is considered in the Voronoi analysis. Don’t choose to drastic values, please. This might lead to unexpected results. One can also refine these parameters later. Moreover, one can also exclude atoms from the computation of the coordination environments (e.g., se=lgf.compute_structure_environments(excluded_atoms=['O'])). Additional parameters such as only_atoms, only_indices and only_cations in combination with valences might be helpful as well.

## Different strategies to analyze the StructureEnvironments¶

Now, the strategy to interpret the data from before is chosen to arrive at information about the coordination environments. One can choose between two different types of strategies: we start with the SimplestChemenvStrategy. This strategy type uses fixed angle and distance parameters for the definition of neighbors in the Voronoi approach. The resulting coordination environment is uniquely defined and is then given as the one with the lowest continuous symmetry measure. One of the disadvantages is that it fails for intermediate coordination environments and depends very much on the cutoff parameters chosen.

Important parameters for this strategy are: distance_cutoff and angle_cutoff. The strategy is correct in about 85% of the cases if one uses distance_cutoff=1.4 and angle_cutoff=0.3. The neighbouring atoms that are considered in this approach have a maximum distance of distance_cutoff × the distance to the closest neighbour and a minimum solid angle of angle_cutoff × the biggest solid angle. For more information, see the code.

Next, one can print the information on the coordination environments for each site in the structure. Here, the information for a site occupied by one oxygen is printed. Please be aware that the counting of the sites starts at 0. In case of doubt, please print the respective site of the structure object with print(structure[isite]) where isite is an Integer.

It will return a dictionary including all relevant information. The item corresponding to the key 'ce_symbol' symbolizes the coordination environment. 'A:2' represents the angular coordination environment. With this strategy, the value of 'ce_fraction' is always equal to 1.0. The 'csm' represents the continous symmetry measure. This value lies between 0.0 and 100.0. At 0.0 the chemical environment in the structure is identical to the model environment and can be interpreted as a distance to a shape. In this example, a 'csm' of 2.26 shows that the environment still shows some similarity to the model environment. Coordination environments with an 'csm' greater than 2.5 are already considered as rather distorted. For more information on the 'csm', have a look at the main text.

A more evolved strategy type, especially for intermediate coordination enviroments, is the MultiWeightsChemenvStrategy. In the following, the default parameters (weigths) are used. Of course, experts can also modify these weights.

This time, two coordination environments for one site exist as indicated by two appearances of 'ce_symbol' and there is also a 'ce_fraction' for each of the environments. The latter indicates the coordination environment is an intermediate between 'A:2' (angular) and 'L:2' (linear) with 67% angular environment and 33% linear environment. Also, there is a 'csm' value for both environments. As already indicated by the 'ce_fraction', the 'csm' for 'A:2' is lower (=in better agreement with the model environment) than the csm for 'L:2'.
The resulting coodination environment is 'T:4' (tetrahedron).