A method for the evaluation of the HF stretch red shift as a function of energy for Ar-n-HF clusters is implemented. The estimator is obtained by extending the canonical ensemble expression to microcanonical simulations. At very low energies the red shift increases linearly with a slope of 0.0452 +/-1 x 10(-4) cm(-1) K-1 for Ar-12-HF and 0.0325 +/-1 x 10(-4) cm(-1) K-1 for Ar-54-HF. The difference in behavior between the two clusters is attributed to differences in isomerizations. These are investigated using a structural comparison algorithm. The core to surface isomerization is evident in the small cluster before melting, though its effects on the physical properties are more subtle than in the canonical ensemble. The HF-Ar exchange isomerizations are practically absent in the larger cluster prior to melting. These findings are in agreement with previous canonical simulations. (C) 2001 American Institute of Physics.