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Atomic structure, specifically for the hydrogen atom, is determined principally by Coulomb interactions among electrons and the nucleus. This leads to the unperturbed energy . There also exist smaller contributions to the energy, most notably from spin-orbit interactions. These are interactions between orbital and spin magnetic moments of the electron, represented by the Hamiltonian

,

where and are the orbital and spin angular momenta, respectively. A secondary perturbation comes from relativistic corrections to electron kinetic energy, represented by a term in the Hamiltonian of the form

.

The total fine structure is then represented by the perturbation

,

with a first-order energy correction

.

This gives the total energy of the state , , to first order in perturbation theory,

,

where is the principal quantum number, is the total electronic angular momentum quantum number and is the fine-structure constant .