- Energy density of crystals, surfaces, slabs, and molecules can be calculated by using
*Density Functional Theory*(DFT). - The absolute energies calculated by the DFT are arbitrary – The “zero” energy can be arbitrarily chosen.
- The relative energies can give useful insight into structural stability (and several other phenomenon).
- In the following example, the “zero” energy is set to be the energy of the tetragonal phase of SnTiO
_{3}at zero strain.

- Polymorphs of SnTiO3 and their relative stability are discussed in detail in following sections: