What is formation energy?

Formation energy (in eV/atom) is the energy change when a compound is formed from its constituent elements in their standard states (e.g., metallic iron, O2 gas, graphite for carbon).

Interpretation

• Negative formation energy: The compound is more stable than the separated elements. Most known stable compounds have negative formation energy.
• More negative = more stable: A formation energy of -3.0 eV/atom indicates a stronger thermodynamic driving force than -1.0 eV/atom.
• Positive formation energy: The compound is energetically unfavorable relative to the elements, though it may still exist as a metastable phase.

Relation to Energy Above Hull

Formation energy alone does not tell you if a compound is the most stable phase at its composition. The energy above hull (Ehull) compares against ALL known competing phases, making it a better stability indicator. A material can have a very negative formation energy but still be above the hull if a more stable phase exists at the same composition.