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openfermion.hamiltonians.fermi_hubbard

Return symbolic representation of a Fermi-Hubbard Hamiltonian.

Used in the notebooks

Used in the tutorials

The idea of this model is that some fermions move around on a grid and the energy of the model depends on where the fermions are. The Hamiltonians of this model live on a grid of dimensions x_dimension x y_dimension. The grid can have periodic boundary conditions or not. In the standard Fermi-Hubbard model (which we call the "spinful" model), there is room for an "up" fermion and a "down" fermion at each site on the grid. In this model, there are a total of 2N spin-orbitals, where N = x_dimension * y_dimension is the number of sites. In the spinless model, there is only one spin-orbital per site for a total of N.

The Hamiltonian for the spinful model has the form

$$ \begin{align} H = &- t \sum{\langle i,j \rangle} \sum{\sigma} (a^\dagger{i, \sigma} a{j, \sigma} + a^\dagger{j, \sigma} a{i, \sigma})

     + U \sum_{i} a^\dagger_{i, \uparrow} a_{i, \uparrow}
                 a^\dagger_{i, \downarrow} a_{i, \downarrow}
    \\
    &- \mu \sum_i \sum_{\sigma} a^\dagger_{i, \sigma} a_{i, \sigma}
     - h \sum_i (a^\dagger_{i, \uparrow} a_{i, \uparrow} -
               a^\dagger_{i, \downarrow} a_{i, \downarrow})
\end{align}

$$

where

- The indices \\(\langle i, j \rangle\\) run over pairs
  \\(i\\) and \\(j\\) of sites that are connected to each other
  in the grid
- \\(\sigma \in \{\uparrow, \downarrow\}\\) is the spin
- \\(t\\) is the tunneling amplitude
- \\(U\\) is the Coulomb potential
- \\(\mu\\) is the chemical potential
- \\(h\\) is the magnetic field

One can also construct the Hamiltonian for the spinless model, which has the form

$$ H = - t \sum_{\langle i, j \rangle} (a^\dagger_i a_j + a^\dagger_j a_i)

    + U \sum_{\langle i, j \rangle} a^\dagger_i a_i a^\dagger_j a_j
    - \mu \sum_i a_i^\dagger a_i.

$$

x_dimension (int): The width of the grid. y_dimension (int): The height of the grid. tunneling (float): The tunneling amplitude \(t\). coulomb (float): The attractive local interaction strength \(U\). chemical_potential (float, optional): The chemical potential \(\mu\) at each site. Default value is 0. magnetic_field (float, optional): The magnetic field \(h\) at each site. Default value is 0. Ignored for the spinless case. periodic (bool, optional): If True, add periodic boundary conditions. Default is True. spinless (bool, optional): If True, return a spinless Fermi-Hubbard model. Default is False. particle_hole_symmetry (bool, optional): If False, the repulsion term corresponds to:

$$
    U \sum_{k=1}^{N-1} a_k^\dagger a_k a_{k+1}^\dagger a_{k+1}
$$


If True, the repulsion term is replaced by:


$$
    U \sum_{k=1}^{N-1} (a_k^\dagger a_k - \frac12)
                       (a_{k+1}^\dagger a_{k+1} - \frac12)
$$


which is unchanged under a particle-hole transformation.
Default is False

hubbard_model An instance of the FermionOperator class.