Hi Scott, It could be an interesting direction, to use these type of lattice calculations to predict, as you suggested, what type of structures (or host compounds, for dopands), will, if not make it zero, which is probably impossible, but minimize third cumulant. Thus, it may be a rational way to design materials, at least hypothetically, with controlled thermal expansion, or even the lack of thereof. I do not care that someone may jump in and patent it, but I will appreciate a Porsche, if possible, when it is licensed. Anatoly ________________________________ From: ifeffit-bounces@millenia.cars.aps.anl.gov on behalf of Scott Calvin Sent: Wed 1/21/2009 10:30 PM To: XAFS Analysis using Ifeffit Subject: Re: [Ifeffit] Cumulant expansion fittings Anatoly, You're right--3 dimensions ruins my symmetry argument. My mistake. On the other hand, I still suspect that there exists a realistic case where forcing the third cumulant to zero cause a much smaller increase in chi-square than forcing the fourth cumulant to zero; e.g., a broad, flat radial distribution function. For those of you out there who are relative novices, this is an entertaining and informative discussion, but I don't want to lose track of the practical point: It is very rare to find a system where the fourth cumulant is both necessary and sufficient. Either the potentials are close enough to harmonic that the fourth cumulant makes little difference, or they are so far from harmonic that the fourth cumulant alone is not enough. --Scott Calvin Sarah Lawrence College On Jan 21, 2009, at 10:11 PM, Frenkel, Anatoly wrote:
Thus, I am pretty much convinced, unless there is some mistake in my reasoning, that no case exists in 3D with a zero third cumulant.
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