Greetings, I am struggling a bit with a non-linear regression. The problem is described below with the known values r and D inidcated. I tried to alter the start values but get always following error message:
Error in nlsModel(formula, mf, start, wts): singular gradient matrix at initial parameter estimates Calls: nls -> switch -> nlsModel I might be missing something with regard to the different algorithms or I just have to try again with some different start values. If anyone finds some time to have a look at that, any advice would be very much appreciated. ##Brice-Model bricemod<-nls(log10(D) ~ log10(Do)*exp(((-4*pi*En*Na)*((ro/2)*(r-ro)^2 +(1/3)*(r-ro)^3))/(R*T)), start=list(Do=0.8, En=390, ro=1.03),trace=TRUE) Na<-6.0221415*10^23 # Avrogadro's number T<-1010 # Temp in K R<-8.3144 #Gas constant [J mol^-1 K^-1] r<-data.matrix(PRr[c("r")]) r La 1.160 Ce 1.143 Pr 1.109 Nd 1.093 Sm1.079 Eu 1.066 Gd 1.053 Tb 1.040 Dy 1.027 Ho 1.015 Er 1.004 Tm 0.994 Yb 0.985 Lu 0.977 D<-data.matrix(PRr[c("D")]) D La 0.1806551 Ce 0.2703113 Pr 0.3757225 Nd 0.5271811 Sm 0.8665835 Eu 1.0812568 Gd 1.0612762 Tb 1.0726612 Dy 1.1679270 Ho 1.1910920 Er 1.1336938 Tm 1.1215107 Yb 0.9619603 Lu 0.8315467 Niklaus -- Niklaus Hürlimann Doctorant-(cand.PhD) Université de Lausanne Institut de Minéralogie et Géochimie L'Anthropole CH-1015 Lausanne Suisse E-mail: niklaus.hurlim...@unil.ch Tel:+41(0)21 692 4452 [[alternative HTML version deleted]]
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