Hubbard model
= Hubbard model
{c}
{title2=$H=H_{\rm hop}+U_H\sum_n n_{n\uparrow}n_{n\downarrow}$}
{wiki}
A lattice <fermion> model combining hopping with local density interaction, conventionally written $U_H\sum_n n_{n\uparrow}n_{n\downarrow}$. Its interaction parameter must be matched when a spin-square representation is used: $-2U(\mathbf S^2-3n/4)$ equals $3U n_\uparrow n_\downarrow$, so $U_H=3U$ for that representation.