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Please explain as simply as possible what the Hubbard-Holtstein model is and what it is used for.

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Could you elaborate on what you want? Also, tell us what you already know... – Manishearth Dec 27 '12 at 15:32
Wait a second, why have I flagged this as unclear what you're asking? Had I gone idiots? +1!. – centralcharge Sep 14 '13 at 12:16

The Hubbard-Holstein model is a electron-phonon model with Hamiltonian

$$ H ~=~ -t\sum_{i{\delta}\sigma}c^{\dagger}_{i\sigma}c^{}_{i+\delta\sigma} +U\sum_{i}n_{i\uparrow}n_{i\downarrow} \nonumber +\omega_{0}\sum_{i}b^{\dagger}_{i}b^{}_{i}+g\sum_{i\sigma}n^{}_{i\sigma}(b^{\dagger}_{i}+b^{}_{i}).$$

Quoting R. Ramakumar, A. N. Das, Polaron cross-overs and d-wave superconductivity in Hubbard-Holstein model, arXiv:cond-mat/0611355,

Here $t$ ($>\,0$) is the hopping energy between molecules at lattice site $ {i}\,$ and its nearest-neighbor lattice sites $i\,+\delta$, $c_{i\sigma}$ ($c_{i\sigma}^{\dagger}$) is the annihilation (creation) operator for the electron with spin $\sigma$ at a lattice site $i$ and $n^{}_{i \sigma}$ is the corresponding number operator, $U$ is on-site Coulomb repulsion, $b^{}_i$ ($b_{i}^{\dagger}$) is the phonon annihilation (creation) operator, and $g$ is the interaction strength.

The Hubbard-Holstein model a hybrid between the Hubbard model (which has no electron-phonon coupling $g=0$), and the Holstein model (which has no Coulomb repulsion $U=0$).

See also p. 60 in the 2003 thesis Phonons, charge and spin in correlated systems by Alexandru Macridin. The pdf file is available here.

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