Hello, I am working from a linux 64 machine on a server with R-2.12 (I can't
update to 2.13). I am iterating through many linear mixed models for
longitudinal data and I occasionally receive the following convergence
error:

> BI.lme <- lme(cd4 ~ time + genBI + genBI:time + C1 + C2 + C11 + C12,
random =~ 1 + time | IID, data = d)

Error in lme.formula(cd4 ~ time + genBI + genBI:time + PC1 +  :
  nlminb problem, convergence error code = 1
  message = false convergence (8)

>From iteration to iteration, genBI is the only variable which changes and it
is a numerical variable equal to 1, 2, or 3 for a given IID.  When I remove
the interaction term genBI:cd4years there is no problem with convergence.
Also, I have played around with scaling time (days to years, eg) which can
solve the problem but then leads to convergence issues in other iterations.
When I shift the time (say in units of years by +2) the model also
converges.  So it seems like the genBI:time is the issue.  Do you have any
suggestions for exploring why I get these convergence errors only on certain
iterations?  I really appreciate any suggestions!

Ben

Here is a summary of the included variables just in case it is helpful:

 IID (808 groups)      cd4              time ( in years)
C1
 ACTG_617 :   26   Min.   :   0.0   Min.   :0.0000   Min.   :-0.032700
 ACTG_1055:   25   1st Qu.: 288.0   1st Qu.:0.4603   1st Qu.: 0.001600
 ACTG_1590:   25   Median : 434.0   Median :1.1534   Median : 0.004300
 ACTG_200 :   23   Mean   : 461.3   Mean   :1.2107   Mean   : 0.002139
 ACTG_420 :   23   3rd Qu.: 599.0   3rd Qu.:1.8630   3rd Qu.: 0.006100
 ACTG_454 :   23   Max.   :2022.0   Max.   :2.7808   Max.   : 0.011300
 (Other)  :11992

         C2                  C11                  C12
Sex
 Min.   :-0.105800   Min.   :0.000   Min.   :0.0000   Min.   :1.000
 1st Qu.: 0.002100   1st Qu.:0.000   1st Qu.:0.0000   1st Qu.:1.000
 Median : 0.007900   Median :0.000   Median :0.0000   Median :1.000
 Mean   : 0.001803   Mean   :0.367   Mean   :0.3020   Mean   :1.095
 3rd Qu.: 0.011700   3rd Qu.:1.000   3rd Qu.:1.0000   3rd Qu.:1.000
 Max.   : 0.030800   Max.   :1.000   Max.   :1.0000   Max.   :2.000

     genBI
 Min.   :1.000
 1st Qu.:1.000
 Median :1.000
 Mean   :1.369
 3rd Qu.:2.000
 Max.   :3.000

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