fit.nlme.viq<-nlme(VIQ~b0+b1*exp(-a*DAYSPC), data=iq, fixed=list(b0~1+sqrt(DCOMA), b1~1+sqrt(DCOMA),a~1), random=list(ID=list(b0~1,b1~1)), control=list(maxIter=100,returnObject=T), start=list(fixed=c(100,-1,-10,-5,0.01)), verbose=T )