Hi Kleber: I think of the Debye model as a model for the stiffness of the bond between two atoms that is like a spring. The vibration depends on the length of the spring (distance between the atoms) and the stiffness of the spring as related through the Debye temperature. Above the Debye temperature the spring is weak and below it is strong. This all works fairly well to describe a symmetrical change in the bond length between the two atoms. It is symmetrical because the bond shortens and lengths by the same amount about a relaxed spring length. A third cummulant adds another freedom in that the amount that the spring shortens is less than the amount that the spring lengthens. This may be the case because the atoms centers repel strongly so the spring may not be able to shorten as much as it is able to lengthen. Very weak bonds like those in solution often have third cummulants. The Debye model does not contain information about a third cummulant. The spring like nature of the bond is not simply related to the skewing of the vibrational amplitude toward larger values. HTH Shelly On Tue, May 11, 2010 at 7:42 AM, Kleber Daum Machado wrote:

I'd like to use the third cumulant option in Artemis, but considering the Debye model. Artemis has a Debye function for the second cumulant (Debye-Waller factor) but not to the third. How could this be done in Artemis?

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