[EXTERNAL] unrealistic values

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Your difficulties probably originate in the complexity of the atomic structures you are working with. ELASTIC_T works best for systems that have either a single local energy minimum (crystalline solid) or a system that quickly samples the full ensemble of minima (liquid). Many solid materials lie between these two extremes. I suggest trying to simplify your problem by either raising or lowering the temperature, or perhaps first doing the zero temperature case with the ELASTIC script.