[R] how to generate and evaluate a design using Algdesign

[sun]

Hi,

  I have some problems when using AlgDesign->optFederov() generating 
designs.

I have 6 variables, all factors. 3^2 and 4^4, I want to have a design that 
can take care of  main effects and two interactions within 2 pair of 
variables v3-v4 and v5-v6, the following is the code

################
require(AlgDesign)
set.seed(1)
levels = c(v1=3,v2=3, v3=4,v4=4,v5=4,v6=4)

dat<-gen.factorial(levels,center=FALSE,varNames=names(levels),factors=
c(1,2,3,4,5,6))

model = ~.+v3:v4+v5:v6

optDgn<-optFederov(model,dat,nRepeat=5,nTrials = 32,criterion = "D",
approximate = F)

----------------------------
this lead to a error msg " nTrials must be greater than or equal to the
number of columns in expanded X" . I thought I do not have that many
columns. if I change approximate to T, this error has gone.

if I remove nTrials argument in function call:
> optDgn<-optFederov(~.+v3:v4+v5:v6,dat,nRepeat=5,criterion = 
> "D",approximate = F)
I got a error : "Singular design."

what would be the cause and what is the sullotion?

another question is, how do I measure or evaluate a design to see if it is 
able to handle which effects(main effects/which intercations)? I got some 
other designs generated by other packages, so I 'd like to check their 
characteristics.

Thanks in advance,

Sun