suppose we have a S = 5/2 spin center with an axial ZFS parameter D = 1 cm-1 and a rhombic ZFS parameter E = 1/3 * D. Suppose also that we specify a D-strain and an E-strain of 0.5 cm-1 and 0 cm-1, respectively. In this case, we will have D values, for which E > 1/3 * D. However, according to the theory, this is not possible. How does EasySpin treat this case?
It is totally possible to have E > D/3 once you fixed your D frame axes xD, yD, zD.
In the presence of a distribution of D and E values, then usually the D frame is chosen based on the means of the distributions and on the 0<E<=D/3 convention. Once that frame is fixed, the distributions in D and E can give E>1/3, based on E = (Dx-Dy)/2.