[R] model formulation in lme4

[François MICHONNEAU]

Hello,

Before to post my message I tried to find the solution by myself but none of the example I 
found match with my problem. It is possible that I did not use the appropriate keywords...

I am trying to analyze results of an experiment took place at 9 stations. A station is 
defined by a combination of Site and Depth (it was an underwater experiment). At each 
station, there was 4 racks. For each rack, 2 treatments were applied on 3 replicates. 
Thus, I would like to fit this model:

Y ~ Site * Depth * Treatment + Site:Depth:Rack + Site:Depth:Rack:Treatment

with Site, Depth and Treatment as fixed factors and Rack as a random factor nested in the 
interaction Site:Depth.

I tried to use lme4 to fit my model. My first trial was (naïvely?):
mdl <- lmer(Y ~ Site * Depth * Treatment + ((Site:Depth)|Rack), abdce.total)

But I obtain a warning message:
"Warning message:
nlminb returned message false convergence (8)
  in: "LMEoptimize<-"(`*tmp*`, value = list(maxIter = 200, tolerance = 
1.49011611938477e-08,  "

Is this message tied to my formula or my data?
Does my formula match with what I would like to do?

Thanks for your help,


François

PS:
 > R.Version()$version.string
[1] "Version 2.3.0 (2006-04-24)"

lme4 0.995-2