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The Hubbard–Stratonovich (HS) transformation is an exact mathematical transformation invented by Russian physicist Ruslan L. Stratonovich and popularized by British physicist John Hubbard. It is used to convert a particle theory into its respective field theory by linearizing the density operator in the many-body interaction term of the Hamiltonian and introducing an auxiliary scalar field. It is defined via the integral identity[1] [2]
![{\displaystyle \exp \left\{-{\frac {a}{2}}x^{2}\right\}={\sqrt {\frac {1}{2\pi a}}}\;\int _{-\infty }^{\infty }\exp \left[-{\frac {y^{2}}{2a}}-ixy\right]\,dy,}](http://wikimedia.org/api/rest_v1/media/math/render/svg/4e85875907eaea481762738d5e2838f777102b4c)
where the real constant . The basic idea of the HS transformation is to reformulate a system of particles interacting through two-body potentials into a system of independent particles interacting with a fluctuating field. The procedure is widely used in polymer physics, classical particle physics, spin glass theory, and electronic structure theory.
The resulting field theories are well-suited for the application of effective approximation techniques, like the mean field approximation. A major difficulty arising in the simulation with such field theories is their highly oscillatory nature in case of strong interactions, which leads to the well-known numerical sign problem. The problem originates from the repulsive part of the interaction potential, which implicates the introduction of the complex factor via the HS transformation.
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