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akre

Austria
5 Posts

Posted - 12/13/2018 : 06:04:35 AM

Origin Ver. and Service Release (Select Help-->About Origin): OriginPro 2018b
Operating System: Win 10

I am trying to fit my data with the Birch-Murnaghan equation which can be seen here: https://en.wikipedia.org/wiki/Birch%E2%80%93Murnaghan_equation_of_state

My data looks as follows

A B
37.69 -.10768897E+02
38.86 -.10818270E+02
40.05 -.10843944E+02
41.26 -.10848027E+02
42.50 -.10832547E+02

I try to create a user defined non-linear curve-fit in xxplicit form, as expression. The independent variable is V, the dependent E. The variables are b,p,v,e. The funcion body is:

e+(9*v*b/16)*{[(v/V)^(2/3)-1]^3*p+[(v/V)^(2/3)-1]^2*[6-4*(v/V)^(2/3]}

Initial values are chosen as 1, the parameters are not fixed and when I do the quick check, I get the error message

E = -- (Math cannot be performed on Text column: )

In the following windows I change nothing and the resulting fit gives me simply nothing. The FitNLCurve values are all "-". I've looked up the error message but found nothing except a hint that "meaningful error messages" are displayed. What am I missing?

YimingChen

1684 Posts

Posted - 12/13/2018 : 09:44:06 AM

Hi,

There are three issues with the equation:
1. Please replace the square brackets and curly brackets to round brackets.
2. the number of brackets are not balanced.
3. v and V can't be distinguished in the equation.

Thanks,
James

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