On 2011-06-30 06:14, Niklaus Hurlimann wrote:
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.
I don't know anything about a Brice-Model, but I doubt that you have
it right. 4*pi*390*Na is about 3*10^27 and that doesn't seem right
to me.
In addition, do put your data into a data.frame (containing
variables r and D) and learn to use dput for posting data.
Peter Ehlers
##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
______________________________________________
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.
