Levels function-orientation of a single crystal

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Levels function-orientation of a single crystal

Postby benceahaver » Mon Feb 12, 2018 8:27 am

Dear Easy Spinner,

I use the levels function for calculation and fit. My crystal has a cubic spacegroup (Fd-3m), but the local point group of the paramagnetic ion is lower (D3d).

How is defined the molecular frame (zM orientation compared to the crystal orientation)? Without this information I do not know how to define properly the angles theta and phi in the argument of the levels function.

My opinion is that the crystal orientation does not matter in this calculation, only the local symmetry of the ion matters: if I assume zM is the local ion principal symmetry axis with respect to the external magnetic field (phi and theta in levels argument), I do not have to use any transformation of frames. Is this right or there is different notation?
This is very important to know because it has a serious effect on the absolute g factor value(s).
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Re: Levels function-orientation of a single crystal

Postby Stefan Stoll » Mon Feb 12, 2018 9:22 am

The local point group of the spin center should be reflected in the symmetry and orientation of the associated spin Hamiltonian parameters. These orientations should be given in Sys.gFrame, Sys.AFrame, etc. They are relative to the molecular frame, which is spin-center fixed. The orientation of the spin center within the crystal is given by Exp.MolFrame which specifies how the spin center lies relative to the crystal axis system. See the page on farmes in the ES docs.

The functions levelsplot and levels are not as sophisticated. For these functions, the angles theta and phi simply give the orientation of the externally applied magnetic field vector in the molecular frame (i.e. the spin-center fixed frame which all the tensors are defined in), disregarding Exp.CrystalOrientation or Exp.CrystalSymmetry.
Stefan Stoll
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