Editing Perturbation theory (section)

===In chemistry===
Many of the [[ab initio quantum chemistry methods]] use perturbation theory directly or are closely related methods. Implicit perturbation theory<ref>{{cite journal | doi = 10.1021/ja00428a004 | title = Theory of the Chemical Bond | year = 1976 | last1 = King | first1 = Matcha | journal = Journal of the American Chemical Society | volume = 98 | issue = 12 | pages = 3415–3420 }}</ref> works with the complete Hamiltonian from the very beginning and never specifies a perturbation operator as such. [[Møller&ndash;Plesset perturbation theory]] uses the difference between the [[Hartree&ndash;Fock]] Hamiltonian and the exact non-relativistic Hamiltonian as the perturbation. The zero-order energy is the sum of orbital energies. The first-order energy is the Hartree&ndash;Fock energy and electron correlation is included at second-order or higher. Calculations to second, third or fourth order are very common and the code is included in most [[Computational chemistry#Software packages|''ab initio'' quantum chemistry programs]]. A related but more accurate method is the [[coupled cluster]] method.