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Nonlinear curve fitting

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[quote][i]Originally posted by hacchaaban[/i]
[br]I have the x and y data and I am trying to fit it to the following function: y = exp(-g/R*x)/(1+exp(-g/R*x), r is the gas constant (8.3145 J/(mol*K)), and g(Gibbs energy- the value that I want to find)is the only unknown parameter
 When creating the function I added x as the independent variable, y as the dependent variable, and R as a constant, and g as the parameter. The data follow an S-form but when I fit the data I get a line for the fit in the bottom om the figure, and not fitted to the data.

Here are my x and y values:

x data:
293,15
294,15
295,15
296,15
297,15
298,15
299,15
300,15
301,15
302,15
303,15
304,15
305,15
306,15
307,15
308,15
309,15
310,15
311,15
312,15
313,15
314,15
315,15
316,15
317,15
318,15
319,15
320,15
321,15
322,15
323,15
324,15
325,15
326,15
327,15
328,15
329,15
330,15
331,15
332,15
333,15
334,15
335,15
336,15
337,15
338,15
339,15
340,15
341,15
342,15
343,15
344,15
345,15
346,15
347,15
348,15
349,15
350,15
351,15
352,15
353,15
354,15
355,15
356,15
357,15
358,15
359,15
360,15
361,15

y values:

1
0,97576
0,98303
0,9886
0,96893
0,971
0,94832
0,9269
0,93655
0,92935
0,91037
0,8979
0,88926
0,8949
0,87848
0,88155
0,88832
0,8721
0,84917
0,85462
0,8394
0,84153
0,81572
0,8052
0,79574
0,7866
0,75916
0,74381
0,71381
0,69659
0,66226
0,61741
0,57457
0,51807
0,45662
0,42624
0,36091
0,33511
0,31287
0,27817
0,27992
0,25211
0,24817
0,22192
0,21553
0,22155
0,21121
0,18497
0,18108
0,16204
0,13555
0,10573
0,0788
0,0761
0,06827
0,06402
0,06007
0,06552
0,0513
0,03245
0,04316
0,02831
0,01766
0,02073
0,02242
0,00783
0,00583
-0,00238
-0,0052

Hope you can help

Hussein

[/quote]

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T O P I C R E V I E W
hacchaaban	Posted - 07/31/2019 : 10:21:56 AM I have the x and y data and I am trying to fit it to the following function: y = exp(-g/Rx)/(1+exp(-g/Rx), r is the gas constant (8.3145 J/(mol*K)), and g(Gibbs energy- the value that I want to find)is the only unknown parameter When creating the function I added x as the independent variable, y as the dependent variable, and R as a constant, and g as the parameter. The data follow an S-form but when I fit the data I get a line for the fit in the bottom om the figure, and not fitted to the data. Here are my x and y values: x data: 293,15 294,15 295,15 296,15 297,15 298,15 299,15 300,15 301,15 302,15 303,15 304,15 305,15 306,15 307,15 308,15 309,15 310,15 311,15 312,15 313,15 314,15 315,15 316,15 317,15 318,15 319,15 320,15 321,15 322,15 323,15 324,15 325,15 326,15 327,15 328,15 329,15 330,15 331,15 332,15 333,15 334,15 335,15 336,15 337,15 338,15 339,15 340,15 341,15 342,15 343,15 344,15 345,15 346,15 347,15 348,15 349,15 350,15 351,15 352,15 353,15 354,15 355,15 356,15 357,15 358,15 359,15 360,15 361,15 y values: 1 0,97576 0,98303 0,9886 0,96893 0,971 0,94832 0,9269 0,93655 0,92935 0,91037 0,8979 0,88926 0,8949 0,87848 0,88155 0,88832 0,8721 0,84917 0,85462 0,8394 0,84153 0,81572 0,8052 0,79574 0,7866 0,75916 0,74381 0,71381 0,69659 0,66226 0,61741 0,57457 0,51807 0,45662 0,42624 0,36091 0,33511 0,31287 0,27817 0,27992 0,25211 0,24817 0,22192 0,21553 0,22155 0,21121 0,18497 0,18108 0,16204 0,13555 0,10573 0,0788 0,0761 0,06827 0,06402 0,06007 0,06552 0,0513 0,03245 0,04316 0,02831 0,01766 0,02073 0,02242 0,00783 0,00583 -0,00238 -0,0052 Hope you can help Br Hussein
3 L A T E S T R E P L I E S (Newest First)
YimingChen	Posted - 07/31/2019 : 2:13:14 PM Hi, I sent you the fdf file. By the way, you need to set the initial value of the fitting parameters as below. James
hacchaaban	Posted - 07/31/2019 : 12:52:01 PM Hi James, thanks a lot. Could you send me the FDF file, or tell me how to add the offset to x, as I get the same when I add xc as a parameter? My email is hacchaaban@gmail.com I am new to origin. Thanks! Hussein
YimingChen	Posted - 07/31/2019 : 10:58:41 AM Hi, It could be that you are missing an offset for x. I replaced x with x-xc, the fitting is more reasonable. see below James

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