# Contact Hyperfine Fields - Some notes¶

Muesr assumes that hyperfine fields are isotropic. The contact term is obtained by specifying two terms: 1) the number of nearest neighbors magnetic atoms to the muon 2) a rcont parameters which governs the maximum radius of the interaction.

Finally, the LocalField object has a ACont property which is an effective hyperfine contact coupling term. The total field is therefore obtained as

The value of N can strongly impact on the results. The best approximation strongly depend on the simulated system and must be considered case by case.

## From CGS to SI¶

Hyperfine couplings are often reported in mol/emu while Muesr uses \({\buildrel _{\circ} \over {\mathrm{A}}}^{-3}\). Here’s how to convert the former into the latter.

Assuming the following formula for the hyperfine contact field produced by the nearest neighboring magnetic atom

where \(A_{cont}\) is expressed in mol/emu and \(\boldsymbol{\mu}_e\) is in emu. As a consequence:

Assuming 1 mol/emu, the value for A in \({\buildrel _{\circ} \over {\mathrm{A}}}^{-3}\) is