Adaptive grid refinement - looping transitions

[jar19]

For the case of looping transitions how does EasySpin determine when to stop subdividing a looping triangle? Then, how does EasySpin handle remaining looping triangles once the threshold has been met, does it simply discard them from the final spectral construction?

[Stefan Stoll]

EasySpin uses adaptive grid refining for modeling the energy level diagram for each sample orientation. It does not use an adaptive strategy over orientations. It uses a fixed orientational grid (determined by Opt.nKnots) and calculates all transitions. For looping transitions, it just adds the spectra from the orientations, rather than using interpolative projection as in the case of non-looping transitions.

[jar19]

This is very helpful, thank you Stefan!

Does this mean that in the case of looping transitions the simple orientational point summation is applied to the entire grid or just to the looping triangles? If the latter is used, does this mean that the weights of the points contributing to a looping triangle are scaled in the point summation to avoid attributing an incorrect weight (too large) to the Voronoi cells?

[Stefan Stoll]

If looping transitions are found, simple summation is applied to all transitions. Weights are of course taken into account correctly.

The resonance fields of looping transitions move really fast as a function of orientation near coalescence points, so due to the finite orientational grid size, there tend to be holes in the simulated spectra at the coalescence points. The only way to reduce these artifacts is to increase the number of orientations.

[jar19]

Thank you for your reply.

I am slightly confused by your reply, please can you clarify the situation? When you stated that the simple summation is applied to all transitions did you mean this is applied only to the (all) transitions of the triangles which contain looping transitions and the 1-D projection is applied to the remaining non-looping triangles, or is the simple summation applied to all transitions at all points on the orientational grid?

When treating spectral construction for the 'simple summation' what method do you use? Is it histogram/Gaussian, gradient broadened or something else?

Out of interest, why did you decide not to implement an adaptive grid via triangle quadrisection (or other method) into EasySpin, is it generally more expensive?

[Stefan Stoll]

If a looping transition is encountered, all transitions (not just the looping one) are accumulated using summation with gradient broadening.

Adaptive quadrisection is currently not implemented, since it gets really complicated for looping transitions.