Hi Scott, Sorry, I'm a little bit confused about what you are recommending for and against. Earlier today you said Well, I do know that many people do it that way, so it's not "wrong." But there are a number of problems with it: and By doing that [not fitting the 1st shell with the 2nd], you're then distorting the fit to the next shell. and Maybe someone who uses this strategy should speak up for it; I'd like to understand what the advantages are. When Jeremy responded, you asked him again what was the advantage of fixing the first shell while fitting the second shell. I read this (past and present form of "read") to mean that you do not approve of this approach. I do not see qualifications about what sorts of spectra you apply these rules to or mentions of special cases But you were eager to hear other opinions because clearly many people use this "problematic" approach...... Now you are saying that you sometimes do fit the 1st shell separately from the 2nd. Strange. It seems like you may have been deliberately provocative in order to get a response, or perhaps there is something else you are trying to get at? Could you please explain what you are recommending? --Matt