EasySpin forum

Hey all,

If I understand correctly, chili should be compatible with the MOMD approach (http://www.sciencedirect.com/science/ar ... 5896901138) described by Budil et. al. However, they describe a Lorentzian linewidth tensor -- Wxx, Wyy, and Wzz in their notation -- which is distinct from the lw or lwpp option in chili:

In addition to the explicit magnetic interactions of the electronic and nuclear spins, it is possible to specify two types of orientation-dependent inhomogeneous broadening: (a) a linewidth tensor W associated with the magnetic frame from rigid-limit spectra and (b) the varying only the dynamic and ordering parameters in the quantities Delta (0) and Delta (2) that specify an added homogeneous linewidth Delta

I haven't been able to find this anywhere in EasySpin, but it seems clear from experimental data (for instance, from the spectra fitted in this paper http://www.nature.com/nmat/journal/v13/ ... at3979.pdf) that this variable can change substantially, and can have a significant influence on the simulation results.

Does anyone know whether fitting for these parameters is possible, or whether there is a way to account for them using chili?

EasySpin only supports an isotropic residual Lorentzian broadening. We'll put a rhombic residual Lorentzian broadening tensor for chili on our to-do list, since I agree that it would be useful to cover all possibilities of other programs. Thanks for pointing this out!

Meanwhile, you could try to work with isotropic residual broadening, but vary the tilt between diffusion tensor and molecular frame beyond just the second Euler angle. Maybe this will improve the fits.

One problem that one faces all the time in multi-component slow-motions spectra is overparamaterization. In the model for a just single spin center, there are already g tensor, A tensor, diffusion tensor, and possible tilts between all these tensors. This is a very large parameter space. Adding a rhombic residual broadening tensor further widens the parameter space and makes it more likely that one ends up in a local minimum where the values of at least some parameters don't make physical sense. E.g, in the paper you cite, there is a 100:1 (3 orders of magnitude difference) between wx and wy for one sample, and 1:2.5 for another. It is hard to see the physical origin of such a stark change, esp. since all the other parameters vary within much smaller and more reasonable ranges.