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The Composition Class

Note

A Composition represents the "recipe" for a chemical. It tells you exactly which elements and how many atoms of each are present, but it does not include any information about where the atoms are located or how they are bonded.

Intro to the Composition class

In chemistry and materials science, we often describe a substance by its formula, such as H2O, Ca2N, or NaCl. In Simmate, these formulas are represented by the Composition class.

Let's load the Composition class:

from simmate.toolkit import Composition

Creating Compositions

You can create a Composition object directly from a formula:

water = Composition("H2O")
salt = Composition("NaCl")

Accessing Composition from Other Classes

The Composition class is actually a fundamental building block. Both the Structure and Molecule classes have a composition property that you can access!

from simmate.toolkit import Structure, Molecule

# For crystals
nacl = Structure.from_file("POSCAR")
print(nacl.composition)

# For molecules
caffeine = Molecule.from_smiles("CN1C=NC2=C1C(=O)N(C(=O)N2C)C")
print(caffeine.composition)

Analyzing Compositions

Once you have a Composition object, you can access many useful properties:

i: Elemental Breakdown

water.elements  # Returns (Element H, Element O)
water.get_el_amt("H")  # Returns 2.0

ii: Formula Reductions

iron_oxide = Composition("Fe2O3")
iron_oxide.reduced_formula  # Returns "Fe2O3"

salt = Composition("Na4Cl4")
salt.reduced_formula  # Returns "NaCl"

iii: Physical Estimates

Simmate's Composition class can even estimate physical properties like density or volume, which is particularly useful when creating random structures:

water.weight  # Atomic weight (approx 18.0)
water.volume_estimate()  # Predicted lattice volume

Comparison

Just like with molecules, compositions can be compared directly to check if two chemicals have the same elemental makeup:

c1 = Composition("H2O")
c2 = Composition("H2O1")
print(c1 == c2)  # Returns True!