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The Structure class

What is a class?

Python "classes" and "objects" are central concepts in python that are important to understand.

In real life, we might say that McDonald's, Burger King, and Wendy's are examples of restauraunts. In python, we could say that mcdonalds, burgerking, and wendys are objects of the class Restaurants.

By organizing objects into classes, python simplifies the way we program. For example, we could design the Restaurants class to have a property called menu. Then, we could view the menu simply by typing We (essentially) set a rule that -- no matter what restaurant we have -- the menu info can be accessed with

This might seem silly, but it becomes very powerful once we want to start building out functionality and anaylses.

Loading the Structure class

In materials science, the class we use most is for crystal structures. In Simmate, we call this class Structure. A crystal structure is always made up of a lattice and a list of atomic sites. Fortunately, this is exactly what we have in our POSCAR file from tutorial 2, so let's use Simmate to create an object of the Structure.

Start by entering this line into the python console and hit enter:

from simmate.toolkit import Structure

This line says we want to use the Structure class from Simmate's code. The Structure class is now loaded into memory and is waiting for you to do something with it.

Loading our structure from a file

Next, make sure you have the correct working directory (just like we did with the command-line). Spyder lists this in the top right -- and you can hit the folder icon to change it. We want to be in the same folder as our POSCAR file. Run this line in your python terminal:

nacl_structure = Structure.from_file("POSCAR")

Here, we told python that we have a Structure and the information for it is located in the POSCAR file. This could be in many other formats, such as a CIF file. But we now have a Structure object and it is named nacl_structure. To make sure it loaded correctly, run this line:


It should print out...

Structure Summary
    abc : 4.02463542 4.02463542 4.02463542
 angles : 59.99999999999999 59.99999999999999 59.99999999999999
 volume : 46.09614820053437
      A : 3.4854365146906536 0.0 2.0123177100000005
      B : 1.1618121715635514 3.2861010599106226 2.0123177100000005
      C : 0.0 0.0 4.02463542
PeriodicSite: Na (0.0000, 0.0000, 0.0000) [0.0000, 0.0000, 0.0000]
PeriodicSite: Cl (2.3236, 1.6431, 4.0246) [0.5000, 0.5000, 0.5000]

This is the same information from our POSCAR!

Manually create a structure in python

Alternatively, we could have created this structure manually:

# note we can name the object whatever we want. We use 's' here.
s = Structure(
        [3.48543651, 0.0, 2.01231771],
        [1.16181217, 3.28610106, 2.01231771],
        [0.0, 0.0, 4.02463542],
    species=["Na", "Cl"],
        [0.0000, 0.0000, 0.0000],
        [0.5000, 0.5000, 0.5000],

Use the structure in a workflow

Whatever your method for creating a structure, we now have our Structure object and we can use its properties (and methods) to simplify our calculations.

For example, we can use it to run a workflow. We did this with the command-line in the last tutorial but can accomplish the same thing with python:

# Using Python (in Spyder)

from simmate.toolkit import Structure
from simmate.workflows.utilities import get_workflow

workflow = get_workflow("")

nacl_structure = Structure.from_file("POSCAR")

# ... modify your structure in some way ...
# (we will learn how in the next section)

result =

If we don't need to modify the structure, we could have just given the filename to our workflow:

from simmate.workflows.utilities import get_workflow

workflow = get_workflow("")
result ="POSCAR")
structure: POSCAR