# Nuclear Spin - Nuclear Spin Coupling in Asymmetric Tops

This object is used to describe the (magnetic)
interactions between a pair of nuclei, and one of these objects is
required for each pair of nuclei that interact. Note that this is
only partly implemented at the moment, and currently requires an I_{12} = I_{1} + I_{2} style coupling
scheme to be used, requiring AsNext to be set to
true for the first nucleus. For some molecules with symmetry this
limits the current implementation to interactions between
equivalent nuclei.

Note that there are multiple definitions of these
terms in the literature. Various different symbols for the scalar
interaction are in use, though the numerical values are the same.
The definition of the dipole - dipole terms are more varied; the
definition used here is as in W. G. Read and W. H. Flygare, JCP **76**,
2238 (1982); doi:
10.1063/1.443297. It differs by a factor of 2 from *S* used for linear molecules.

## Parameters

Note that where *x*, *y* and *z*
axes are referred to below, the mapping to *a*, *b*
and *c* axes is determined by the representation - see Symmetry and Axis Systems so that, for
example, `D``zz` corresponds to *D*_{aa}
in a I*r* representation but *D*_{cc} in a III*r*
representation.

`J` |
Scalar nuclear spin - nuclear spin interaction, **I**_{1}.**I**_{2}. |

`Dzz` |
*D*_{zz} = -(*D*_{xx}+*D*_{yy})
component of nuclear spin - nuclear spin dipole dipole
interaction. |

`Dxxmyy` |
*D*_{xx}-*D*_{yy} component of
nuclear spin - nuclear spin dipole dipole interaction. |

`Dab` |
ab component of nuclear spin - nuclear spin dipole dipole
interaction. |

`Dac` |
ac component of nuclear spin - nuclear spin dipole dipole
interaction. |

`Dbc` |
bc component of nuclear spin - nuclear spin dipole dipole
interaction. |