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Screensize: 640 x 480 800 x 600 1024 x 768 1280 x 1024 UserName: Password: Anti-Spam Code: Format Mode: Basic  Help  Prompt  Format: Font Andale Mono Arial Arial Black Book Antiqua Century Gothic Comic Sans MS Courier New Georgia Impact Lucida Console Script MT Bold Stencil Tahoma Times New Roman Trebuchet MS Verdana   Size 1 2 3 4 5 6   Color Black Red Yellow Pink Green Orange Purple Blue Beige Brown Teal Navy Maroon LimeGreen Message: * HTML is OFF * Forum Code is ON Smilies [quote][i]Originally posted by akre[/i] [br]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? [/quote]   Check here to include your profile signature. Check here to subscribe to this topic.