Hello all,

after being inspired by the John and Robert groups great work with stacking fault simulation using TOPAS, I have tried it on a structure that has been irritating me for a while. I'm afraid I cannot mention which, due to "intellectual property" issues but I hope I can reveal enough to illustrate my problem.

5 polymorph layers based on a sublayer. Each of the polymorphs is described by a stack of 2-4 sublayers with differing stacking vectors. To demonstrate my issue a tiny subset of probabilities will do:

#list P12 P13 P14 P15 P23 P24 P25 P34 P35 P45
{
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
0.0 0.0 0.0 0.1 0.0 0.1 0.0 0.0 0.0 0.0
0.0 0.0 0.0 0.1 0.1 0.0 0.0 0.1 0.0 0.0
0.0 0.0 0.0 0.2 0.0 0.2 0.0 0.0 0.0 0.0
}

and I use 200 sequences of 200 layers to write out a pattern

num_runs 4
iters 0
macro Nseq { 200 }
macro Nv { 200 }
yobs_eqn aac.xy = 1;
min 5
max 40
del 0.02
Out_X_Ycalc(diff##Run_Number##.txt)

Here are my stack definitions. As you can see, I have tried to avoid probabilities of 0. I have constrained the probability of of one polymorph stacking on the identical polymorph to be the same (P11). Not sure that is sensible at the moment, but it does limit the possibilities.

seed
generate_stack_sequences
{
number_of_sequences Nseq
number_of_stacks_per_sequence Nv

prm !layer_height 5.0

prm Probability_12 = If(P12(Run_Number)==0,0.001,P12(Run_Number));:0.00100
prm Probability_13 = If(P13(Run_Number)==0,0.001,P13(Run_Number));:0.00100
prm Probability_14 = If(P14(Run_Number)==0,0.001,P14(Run_Number));:0.00100
prm Probability_15 = If(P15(Run_Number)==0,0.001,P15(Run_Number));:0.20000
prm Probability_23 = If(P23(Run_Number)==0,0.001,P23(Run_Number));:0.00100
prm Probability_24 = If(P24(Run_Number)==0,0.001,P24(Run_Number));:0.20000
prm Probability_25 = If(P25(Run_Number)==0,0.001,P25(Run_Number));:0.00100
prm Probability_34 = If(P34(Run_Number)==0,0.001,P34(Run_Number));:0.00100
prm Probability_35 = If(P35(Run_Number)==0,0.001,P35(Run_Number));:0.00100
prm Probability_45 = If(P45(Run_Number)==0,0.001,P45(Run_Number));:0.00100
prm P11 = 1.0 - Probability_12 - Probability_13 - Probability_14 - Probability_15;:0.79700

'Polymorph 1
Transition(A, A, layer_height)
to B = 1; a_add = 0; b_add = 0;
Transition(B, A, layer_height)
to A = P11 ; a_add = 0; b_add = 0;
to C = Probability_12; a_add = 0; b_add = 0;
to F = Probability_13; a_add = 0; b_add = 0;
to K = Probability_14; a_add = 0; b_add = 0;
to P = Probability_15; a_add = 0; b_add = 0;

'Polymorph 2
Transition(C, A, layer_height)
to D = 1; a_add = 0; b_add = -1/3;
Transition(D, A, layer_height)
to E = 1; a_add = -1/3; b_add = 0;
Transition(E, A, layer_height)
to A = Probability_12; a_add = 0; b_add = 0;
to C = P11 ; a_add = 0; b_add = 0;
to F = Probability_23; a_add = 0; b_add = 0;
to K = Probability_24; a_add = 0; b_add = 0;
to P = Probability_25; a_add = 0; b_add = 0;

'Polymorph 3
Transition(F, A, layer_height)
to G = 1; a_add = 0; b_add = -1/3;
Transition(G, A, layer_height)
to H = 1; a_add = -1/3; b_add = 0;
Transition(H, A, layer_height)
to I = 1; a_add = 0; b_add = 1/3;
Transition(I, A, layer_height)
to J = 1; a_add = -1/3; b_add = 0;
Transition(J, A, layer_height)
to A = Probability_13; a_add = 0; b_add = 0;
to C = Probability_23; a_add = 0; b_add = 0;
to F = P11 ; a_add = 0; b_add = 0;
to K = Probability_34; a_add = 0; b_add = 0;
to P = Probability_35; a_add = 0; b_add = 0;

'Polymorph 4
Transition(K, A, layer_height)
to L = 1; a_add = -1/3; b_add = 0;
Transition(L, A, layer_height)
to M = 1; a_add = 0; b_add = -1/3;
Transition(M, A, layer_height)
to N = 1; a_add = 1/3; b_add = 0;
Transition(N, A, layer_height)
to O = 1; a_add = 0; b_add = 1/3;
Transition(O, A, layer_height)
to A = Probability_14; a_add = 0; b_add = 0;
to C = Probability_24; a_add = 0; b_add = 0;
to F = Probability_34; a_add = 0; b_add = 0;
to K = P11 ; a_add = 0; b_add = 0;
to P = Probability_45; a_add = 0; b_add = 0;

'Polymorph 5
Transition(P, A, layer_height)
to Q = 1; a_add = 1/3; b_add = 0;
Transition(Q, A, layer_height)
to R = 1; a_add = 0; b_add = -1/3;
Transition(R, A, layer_height)
to S = 1; a_add = -1/3; b_add = 0;
Transition(S, A, layer_height)
to T = 1; a_add = 0; b_add = 1/3;
Transition(T, A, layer_height)
to A = Probability_15; a_add = 0; b_add = 0;
to C = Probability_25; a_add = 0; b_add = 0;
to F = Probability_35; a_add = 0; b_add = 0;
to K = Probability_45; a_add = 0; b_add = 0;
to P = P11 ; a_add = 0; b_add = 0;
}

Now to the problem :)
With two polymorph layers the model works perfectly.
With three it randomly throws "max stack allowed error"
With more than three it always throws the error.

Fiddling with the number of layers and number of sequences does not seem to influence the matter too much.

I am using the current commercial version 6 of TOPAS.

Any help will be greatly appreciated.

Greetings from Ludwigshafen,
Bernd