%       Copyright (c) 2000 Bruce Ravel
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%       Using FEFF and FEFFIT, Part 1: Presentation", "EXAFS Analysis
%       Using FEFF and FEFFIT, Part 2: Commentary", and all related
%       analysis examples, with no Front-Cover Texts, and with no
%       Back-Cover Texts.  A copy of the license is included in the
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% 
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% and FEFFIT') are free documentation; you can redistribute it and/or
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title =  GaN, epsilon parallel to surface normal

data = data/parak.chi
out = fits/para
kspout = yes	rspout = yes	qspout = no
kmin = 2	kmax = 12	dk = 1		kw = 1
rmin = 1.0	rmax = 3.4
format = ascii	all = no

% bkg = true
bkgfile = fits/parak.background

%% This fitting problem is not to much more complicated, at least out
%% to the second shell, than copper metal.  Three interesting things
%% are introduced in this fit:
%%  1. polarization:  separate feff calulations were run for parallel
%%     and perpendicular polarization.  The appropriate set of
%%     feffNNNN.dat files are used.
%%  2. multiple e0 parameters:  this will be discussed in more detail
%%     when we look at EuTiO3.  The basic idea is that we want to
%%     account for errors in feff's phases which might be element
%%     specific.  Thus an e0 parameter is used for each species of
%%     back-scatterer.
%%  3. path lengths are computed from crystallographic parameters:
%%     below we set and guess various parameters relating to the
%%     crystal structure and use math expressions to come with delr
%%     math expressions.  We will discuss this sort of model building
%%     in much greater detail when we look at PbTiO3.
%%
%% The main thing to take away from this exercise is that polarized
%% data and non-polarized data are handled in the same manner by
%% feffit, assuming that feff has made the proper polarized
%% calculation.  This is much easier and more accurate than using a
%% cos^2 rule to modify path amplitudes.
%%
%% Things to try:
%%
%%   1.  Uncomment the nofit keyword below and run feffit.  This will
%%       tell feffit to read and interpret the input file and all data
%%       and feff files, but not to run the fit.  The log and prm
%%       files will contain the results of all math expressions
%%       evaluated with the starting guess values.  This is a good way
%%       to "spell check" math expressions.  All delr values should be
%%       0 within numerical precision (a few times 10^-5 for feffit,
%%       depending on the math expression).
%%   2.  You could try adding higher shells and doing the fit.  Beyond
%%       the second shell GaN is a bit tricky and the data isn't quite
%%       good enough to properly determine the structure out there.
%%       Still, it is good to get some practice with MS paths and the
%%       fit results shouldn't be that bad...


%==+==+==+==+==+==+==+==+==+==+==+==+==+==+==+==+==+==+==+==+==+==+==

% nofit

set	amp	0.93

guess	sig_n	0.001
guess	sig_ga	0.004
% guess	sig_n2	0.001
% guess	sig_n3	0.001
set	sigmm	0.00042

guess	e0n	0.0
guess	e0ga	0.0

set	ei	0.0

%==+==+==+==+==+==+==+==+==+==+==+==+==+==+==+==+==+==+==+==+==+==+==

set	a0	3.190
set	delta_a	0
set	a	a0+delta_a

set	c0	5.189
set	delta_c	0
set	c	c0+delta_c

guess	u	0.377

set	r_2	sqrt(a**2/3 + ((.5-u)*c)**2)	! N 1st
set	r_3	sqrt(a**2/3 + (c/2)**2)		! Ga 2nd
% set	r_6	sqrt(4*a**2/3 + ((0.5-u)*c)**2)	! shorter N 3rd shell
% set	r_7	sqrt( a**2 + (u*c)**2 )		! longest N 3rd shell

%==+==+==+==+==+==+==+==+==+==+==+==+==+==+==+==+==+==+==+==+==+==+==

path	1	feff/para/feff0001.dat
id	1	amp=100.000  deg=1  nlegs=2  reff=1.9372
e0	1	e0n
delr	1	u*c - reff
sigma2	1	sig_n + sigmm
ei	1	ei
s02	1	amp

path	2	feff/para/feff0002.dat
id	2	amp=26.776  deg=3  nlegs=2  reff=1.9462
e0	2	e0n
delr	2	r_2 - reff
sigma2	2	sig_n + sigmm
ei	2	ei
s02	2	amp

path	3	feff/para/feff0003.dat
id	3	amp=100.000  deg=6  nlegs=2  reff=3.1690
e0	3	e0ga
delr	3	r_3 - reff
sigma2	3	sig_ga + sigmm
ei	3	ei
s02	3	amp



!!&& Local Variables:
!!&& input-out-path: "fits/"
!!&& input-feff-path: "feff/"
!!&& input-data-path: "data/"
!!&& input-program-name: "feffit"
!!&& End:
%# Input-mode Time-stamp: <2000/03/11 21:09:45 bruce>