[Ifeffit] Questions for Artemis fit

[Scott Calvin]

Hi Yanyun,

On Aug 19, 2014, at 12:05 PM, huyanyun at physics.utoronto.ca wrote:
> 
> But my question is, for each path there are a set of Path Parameters,  
> does that mean we have to fit this set of path parameters independent  
> from those used in other paths?  For instance, if I am going to  
> include 30 paths in my fitting, do I need to guess about 150  
> (30*5=150) parameters (except guess the same 'enot' for all paths)?

No. One of the great principles of Ifeffit (and thus Artemis) is that the path parameters do not have to be the same as the fit parameters.

As a simple example, it's good to start with the model that all paths have the same value for E0. So you can guess a single parameter and then use that parameter for the E0 for all paths.

As another simple example, a cubic crystal might be modelled as having a uniform thermal (or Vegard's law) expansion. Then there could be a single guessed parameter indicating the fractional expansion (called, for example, alpha) while the delr for each path could then be entered as alpha*reff.

Much more complicated constraint schemes are possible, but for fits with large number of paths, the number of guessed parameters is almost always much much lower than the number of path parameters.

> 
> As shown on the fit Log file, the Correlation value between two fit  
> parameters is the bigger the better, or the smaller the better?

Not necessarily. Correlations are provided to help understand the relationship between guessed parameters, but there's nothing inherently wrong with a high correlation...the uncertainty associated with that relationship is already represented in the uncertainties reported with the fit. In other words, it is more direct to focus on getting the uncertainties to be lower, rather than the correlations.

--Scott Calvin
Sarah Lawrence College