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 I₁₂ = I₁ + I₂ 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, Dzz corresponds to D_aa in a Ir representation but D_cc in a IIIr representation.

`J`	Scalar nuclear spin - nuclear spin interaction, I₁.I₂.
`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.