CODEWITHSUNDEEP
gnuplot
Vijay
Vijay

Reputation: 985

Fitting in gnuplot with three variables

I am trying to fit some data using gnuplot.

Here is the data (variables h, k,l and I): 

#h  k   l   I

2   1   1   7807
2   2   0   9664
3   2   1   6042
4   0   0   1394
3   3   2   1358
4   2   0   4896

### Function
I(h,k,l) = M * (F * ( (sin(A*pi*sqrt(h*h+k*k+l*l)*L))/(A*2*pi*sqrt(h*h+k*k+l*l)) ))^2

### Initial values
M=1
F=0.5
A=1
L=1

### Fitting 
fit I(h,k,l) "cavendish.data" using 1:2:3 via M, F, A, L

I want to determine the constants M,F,A and L from this fitting.

When I run this code I get message undefined variable: h

How can I could determine the variables. Thanks in advance.

Upvotes: 0

Views: 1732

Answers (1)

Tobias Müller
Tobias Müller

Reputation: 605

Try using a recent version of gnuplot (>= 5.0), which supports fit commands with more than two variables (see release notes). Also note, that the power operator in gnuplot is ** and not ^.

You're example has to be changed slightly to work: 

### Function
I(h,k,l) = M * (F * ((sin(A*pi*sqrt(h*h+k*k+l*l)*L))/(A*2*pi*sqrt(h*h+k*k+l*l)) ))**2

### Initial values
M=1.0
F=0.5
A=1.0
L=1.0

### Fitting
set dummy h, k, l
fit I(h,k,l) "cavendish.data" using 1:2:3:4 via M, F, A, L

Upvotes: 1

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