What is Molecular Frame in a Crystal?

[usiansumo]

I am very new to easyspin. I work on single crystals. I know while simulating energy levels we can use
levelsplot(mySystem,Ori,FieldRange,Freq); where "Ori can be either an array of numbers or a string. As array, it specifies the Euler angles [phi theta] or [phi theta chi] of the magnetic field relative to the molecular frame of the spin system" . But, in single crystal what is molecular frame? Is it defect frame?

[Stefan Stoll]

Yes. The molecular frame is the frame in which the spin center (defect etc.) is defined.

[usiansumo]

Thank you