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(a) The definition of the angles ϕ and ψ defining the relative orientation in the Er2 complex with equivalent metal sites. (b) Variation of 
with respect to ϕ and ψ. The internal dashed line corresponds to the angle π/4 and the external to the angle π/2 between the magnetic axes. The blue and the red regions stand for the negative and positive values of 
, respectively (cm−1).
