Editing Hartree–Fock method (section)

===Hartree–Fock===
A solution to the lack of anti-symmetry in the Hartree method came when it was shown that a [[Slater determinant]], a [[determinant]] of one-particle orbitals first used by Heisenberg and Dirac in 1926, trivially satisfies the [[exchange symmetry|antisymmetric]] property of the exact solution and hence is a suitable [[ansatz]] for applying the [[variational principle]]. The original Hartree method can then be viewed as an approximation to the Hartree–Fock method by neglecting [[exchange symmetry|exchange]]. Fock's original method relied heavily on [[group theory]] and was too abstract for contemporary physicists to understand and implement. In 1935, Hartree reformulated the method to be more suitable for the purposes of calculation.<ref name="Hartree1935">{{cite journal |first1=D. R. |last1=Hartree |first2=W. |last2=Hartree |title=Self-consistent field, with exchange, for beryllium |journal=[[Proceedings of the Royal Society A]] |volume=150 |issue=869 |pages=9 |year=1935 |doi=10.1098/rspa.1935.0085 |bibcode=1935RSPSA.150....9H |doi-access=free }}</ref>

The Hartree–Fock method, despite its physically more accurate picture, was little used until the advent of electronic computers in the 1950s due to the much greater computational demands over the early Hartree method and empirical models.<ref>{{cite journal | url=https://link.aps.org/doi/10.1103/PhysRev.81.385 | doi=10.1103/PhysRev.81.385 | title=A Simplification of the Hartree-Fock Method | year=1951 | last1=Slater | first1=J. C. | journal=[[Physical Review]] | volume=81 | issue=3 | pages=385–390 | bibcode=1951PhRv...81..385S | url-access=subscription }}</ref> Initially, both the Hartree method and the Hartree–Fock method were applied exclusively to atoms, where the spherical symmetry of the system allowed one to greatly simplify the problem. These approximate methods were (and are) often used together with the [[central field approximation]] to impose the condition that electrons in the same shell have the same radial part and to restrict the variational solution to be a [[Spin (physics)#Mathematical formulation|spin eigenfunction]]. Even so, calculating a solution by hand using the Hartree–Fock equations for a medium-sized atom was laborious; small molecules required computational resources far beyond what was available before 1950.