Question

zero field splitting

Answer 1

Zero field splitting describes various interactions of the energy levels of an electron spin (S>1/2) even in the absence of an applied magnetic field. It is important in the electron spin resonance of biological molecules.
ZFS parameters originate from electron-electron dipolar interaction, and from spin/orbit coupling. Last contribution is dominant in metal centers with S>1/2. For Iron, for example, the term can be even close to cm-1.
In general the term depends on the crystal field intensity, on the symmetry of the coordination...