I think that these aren't good initial values. If you do a plot of data and add a curve, the curve don't approximate the data. Frequently I use interactive procedures to get good initial values. Using playwith() you can handle sliders to adjust values and use in nls(), look the following
r <- c(1.16,1.143,1.109,1.093,1.079,1.066,1.053,1.040,1.027,1.015,1.004,0.994,0.985,0.977) D <- c(0.1806551,0.2703113,0.3757225,0.5271811,0.8665835,1.0812568,1.0612762,1.0726612, 1.167927,1.191092,1.1336938,1.1215107,0.9619603,0.8315467) names <- c(La,Ce,Pr,Nd,Sm,Eu,Gd,Tb,Dy,Ho,Er,Tm,Yb,Lu) plot(log(D)~r) Do <- 0.8; En <- 390; ro <- 1.03 curve(log10(Do)*exp(((-4*pi*En*Na)*((ro/2)*(x-ro)^2+(1/3)*(x-ro)^3))/(R*T)), add=TRUE) Na <- 6.0221415*10^23 # Avrogadro's number T <- 1010 # Temp in K R <- 8.3144 # Gas constant [J mol^-1 K^-1] ## 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) require(playwith) start <- list() playwith( { plot(log(D)~r) curve(log10(Do)*exp(((-4*pi*En*Na)*((ro/2)*(x-ro)^2+(1/3)*(x-ro)^3))/(R*T)), add=TRUE) start <<- list(Do=Do, En=En, ro=ro) }, parameters=list( Do=seq(0.7,0.8,by=0.001), En=seq(350,450,by=1), ro=seq(0.5,1.5,by=0.1))) start bricemod <- nls(log10(D)~log10(Do)*exp(((-4*pi*En*Na)*((ro/2)*(r-ro)^2+(1/3)*(r-ro)^3))/(R*T)), start=start, trace=TRUE) Bests. Walmes. ========================================================================== Walmes Marques Zeviani LEG (Laboratório de Estatística e Geoinformação, 25.450418 S, 49.231759 W) Departamento de Estatística - Universidade Federal do Paraná fone: (+55) 41 3361 3573 VoIP: (3361 3600) 1053 1173 e-mail: wal...@ufpr.br twitter: @walmeszeviani homepage: http://www.leg.ufpr.br/~walmes linux user number: 531218 ========================================================================== [[alternative HTML version deleted]]
______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.