.. _ug_tb_sh_2-1:

*************
(2,  1) terms
*************

First type of the bilinear terms, in which both spin operators are associated with the
same site.

.. math::
    C_{2,1}
    \sum_{\substack{\mu, \\ \alpha_1, \\ i_1, i_2}}
    J^{i_1, i_2}_{0; \alpha_1, \alpha_1}
    S_{\mu, \alpha_1}^{i_1}
    S_{\mu, \alpha_1}^{i_2}

Relevant API
============

*   :py:attr:`magnopy.Convention.c21`

    Convention constant.

*   :py:meth:`magnopy.SpinHamiltonian.add`

    Method to add the parameter to the Hamiltonian.

*   :py:meth:`magnopy.SpinHamiltonian.remove`

    Method to remove the parameter from the Hamiltonian.

*   :py:attr:`magnopy.SpinHamiltonian.p21`

    An iterator over the parameters already added to the Hamiltonian.