The double peak

Dear Nonmem users, I am working with a double peaks PK model. I refered to a recent question about this topic and tried the double absorption compartment model suggested by somebody. Howerver, in my real data, the Ka and Ke for each peak are different. The ka of the first peak is greater than that of the second, and the ke of the first is also greater than the second. Therefore, the profile has a sharp peak at first, then a flat peak after that. If I use the double absorption compartment model, it can give me two different Ka, but can't give me two different Ke in different time. Then I wanted to add a periphal compartment to solve this problem. My idea is after the admin, part of drug(F1) gets into the plasma quickly(Ka1) then quickly goes to the periphal. So there will be a big Kcp (from central to periphal). In the same time, drug eliminates to outside from plasma in a speed of Ke, which is very small. After a period of time, the drug in periphal begins to flow back to central in a same speed of Kcp( Kpc=Kcp). This will retain the drug concentration in the plasma and prevent the fast drop after the second peak in the c-t profile. Also, a sencond part of drug(F2) will be absorbed with a slower Ka2 after a proper lag time. Finally, the drug will eliminate outside in the speed of Ke, which is very slow. In order to do this, I use lag time between periphal and central. I am not sure if it is permitted by NONMEM, but at least I didn't find error message when I did simulation. Also, I don't know if my model can really work as I expected. But when I did the estimation, it crashed. Would someone have some good idea about this or can tell me if there is any wrong in my script? Thank you in advance! My model is : $DATA ****.csv $SUBROUTINES ADVAN6 TRANS1 TOL=3 $MODEL NCOMP=4 COMP=(ABS1) COMP=(ABS2) COMP=(CENTRAL) COMP=(PERI) $PK K13 =THETA(1) K23 =THETA(2) KE =THETA(3)*EXP(ETA(1)) ALAG2=THETA(4)*EXP(ETA(2)) F1 = THETA(5) F2 = 1- F1 K34 =THETA(6)*EXP(ETA(3)) K43 =THETA(7)*EXP(ETA(4)) ALAG4=THETA(8)*EXP(ETA(5)) $DES DADT(1)=-K13*A(1) DADT(2)= -K23*A(2) DADT(3)= K13*A(1)+K23*A(2)-KE*A(3)-K34*A(3)+K43*A(4) DADT(4)= K34*A(3)-K43*A(4) $ERROR IPRED= F Y = IPRED * (1+ERR(1)) + ERR(2) IRES = DV - F IWRES = IRES/IPRED $THETA 1 FIX ; K13 0.6 FIX ; K23 (0.0001,0.001,0.01) ; KE (20,40,60) ; ALAG2 0.7 FIX ; F1 (0.5,0.75,1) ; K34 (0.5,0.75,1) ;K43 (20,40,60) ;ALAG4 $OMEGA 0.2 0.1 0.1 0.1 0.1 $SIGMA 0.1 0.1 $ESTIMATION METHOD=1 PRINT=1 MAXEVAL=9999 NOABORT SIGDIGITS=3 POSTHOC INTERACTION MSFO=msfo.outputfile ................ Maurice