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How do I fix negative values in a function to zero ?

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I have the following script and function with a code to fix the negative values to zero but it is failing for some reason. What could be the issue?
%Script:
%initial conditions
y0=[18.16;0.52;25.64;172.6;179.48;0.0005;2;0.0005;55;50;0.5];
tSpan=[0 535];
M1=eye(11);
options = odeset('RelTol',1e-20,'AbsTol',1e-10,'Mass',M1,'MassSingular', 'maybe','MStateDependence','strong','InitialStep',1e-10, 'MStateDependence','strong');
[tSol, ySol]=ode15s(@(t,y) MBBR12(t,y), tSpan, y0, options);
%Funtion as follows:
function fval=MBBR12(t,y)
%Define the three variables
Xaob=y(1);
Xnb=y(2);
Xnsp=y(3);
Xamx=y(4);
Xhan=y(5);
Xs=y(6);
Sse=y(7);
Sno3=y(8);
Sno2=y(9);
Snh4=y(10);
So2=y(11);
T=303;
%aob
O1=68/(0.00831*293*(273+T)); u1=0.054*exp(O1*(T+293)); Ko2aob=0.6; Knh4aob=2.4; Yaob=0.15; Kno3aob=0.5;
%nb
O2=44/(0.00831*293*(273+T)); u2=0.047*exp(O2*(T+293)); Ko2nb=2.2; Kno2nb=0.5*1.375; Ynb=0.041;Kno3nb=0.5;
%nsp
O3=44/(0.00831*293*(273+T)); u3=0.047*exp(O3*(T+293)); Ko2nsp=2.2; Kno2nsp=5.5; Ynsp=0.041;Kno3nsp=0.5;
%amx
O5=71/(0.00831*293*(273+T)); u5=0.0022*exp(O5*(T+293)); Kno2amx=0.05; Knh4amx=0.07; Yamx=0.159; Ko2amx=0.01; Kno3amx=0.5;
%han
O6=0.069;u6=7.2*exp((T+293)*0.069); KSsehan=2; Kno2han=0.5;Kno3han=0.5; YNo2han=0.3; Ko2han=0.2;
YNo3han=0.43; Yhaer=0.54;
%other constants
baob=0.0054*exp(O6*(T+293)); bnb=0.0025*exp(O6*(T+293));bnsp=bnb; bamx=0.00013*exp(O6*(T+293)); bhan=0.008*exp(O6*(T+293));
INBM=0.07; fI=0.1; namx=0.5; nhan=0.6;naob=0.5; nnb=0.5; nnsp=0.5;
KX=1; KH=0.125*exp(0.069*(T+293)); INXI=0.02;
% input parameters
Sseo=300; Sno3o=0.2; Qo=75; Xso=250;
Snh4o=650; Sno2o=0.2; So2o=0;
%oxygen transfer parameters
KaL=80;
V=1400; SRT=40;
%fix output values to zero
Sno3(Sno3<0)=0;Xaob(Xaob<0)=0;Xnb(Xnb<0)=0;Xnsp(Xnsp<0)=0;Xamx(Xamx<0)=0;Xhan(Xhan<0)=0;Xs(Xs<0)=0;
Sse(Sse<0)=0;Sno2(Sno2<0)=0;Snh4(Snh4<0)=0;So2(So2<0)=0;
% main functions CSTR 1
fval=[((0-V*Xaob/SRT)/V + u1*(So2/(Ko2aob+So2))*(Snh4/(Knh4aob+Snh4))*Xaob - baob*(So2/(Ko2aob+So2))*Xaob - baob*naob*((Ko2aob/(Ko2aob+So2))*(Sno2+Sno3)/(Kno3aob+Sno2+Sno3))*Xaob)
((0-V*Xnb/SRT)/V + u2*(Sno2/(Kno2nb+Sno2))*(So2/(Ko2nb+So2))*Xnb - bnb*(So2/(Ko2nb+So2))*Xnb - bnb*nnb*((Ko2nb/(Ko2nb+So2))*(Sno2+Sno3)/(Kno3nb+Sno2+Sno3))*Xnb)
((0-V*Xnsp/SRT)/V + u3*(Sno2/(Kno2nsp+Sno2))*(So2/(Ko2nsp+So2))*Xnsp - bnsp*(So2/(Ko2nsp+So2))*Xnsp - bnsp*nnsp*((Ko2nsp/(Ko2nsp+So2))*(Sno2+Sno3)/(Kno3nsp+Sno2+Sno3))*Xnsp)
((0-V*Xamx/SRT)/V + u5*((Snh4/(Knh4amx+Snh4))*(Sno2/(Kno2amx+Sno2))*(Ko2amx/(Ko2amx+So2)))*Xamx-bamx*(So2/(Ko2amx+So2))*Xamx - bamx*namx*((Ko2amx/(Ko2amx+So2))*(Sno2+Sno3)/(Kno3amx+Sno2+Sno3))*Xamx)
((0-V*Xhan/SRT)/V + (u6*(Sse/(KSsehan+Sse))*(Sno2/(Kno2han+Sno2))*(Ko2han/(Ko2han+So2)) + u6*(Sse/(KSsehan+Sse))*(Sno3/(Kno3han+Sno3))*(Ko2han/(Ko2han+So2))+u6*((Sse/(KSsehan+Sse))*(So2/(Ko2han+So2)))*Xhan)- bhan*(So2/(Ko2han+So2))*Xhan - bhan*nhan*((Ko2han/(Ko2han+So2))*(Sno2+Sno3)/(Kno3han+Sno2+Sno3))*Xhan)
((Qo*Xso-Qo*Xs)/V - KH*((Xs/Xhan)/(KX+(Xs/Xhan)))*Xhan)
((Qo*Sseo-Qo*Sse)/V - (u6*nhan*(1/YNo2han)*(Sse/(KSsehan+Sse))*(Sno2/(Kno2han+Sno2))*(Ko2han/(Ko2han+So2)))*Xhan -(u6*nhan*(1/YNo3han)*(Sse/(KSsehan+Sse))*(Sno3/(Kno3han+Sno3))*(Ko2han/(Ko2han+So2)))*Xhan - ((u6/Yhaer)*(Sse/(KSsehan+Sse))*(So2/(Ko2han+So2))*Xhan) + KH*((Xs/Xhan)/(KX+(Xs/Xhan)))*Xhan)
((Qo*Sno3o-Qo*Sno3)/V - (u6*nhan*((1-YNo3han)/(2.86*YNo3han))*(Sse/(KSsehan+Sse))*(Sno3/(Kno3han+Sno3))*(Ko2han/(Ko2han+So2)))*Xhan + (u5/1.14)*(Snh4/(Knh4amx+Snh4))*(Sno2/(Kno2amx+Sno2))*(Ko2amx/(Ko2amx+So2))*Xamx + (u2/Ynb*(Sno2/(Kno2nb+Sno2))*(So2/(Ko2nb+So2))*Xnb) + ((u3/Ynsp)*(Sno2/(Kno2nsp+Sno2))*(So2/(Ko2nsp+So2))*Xnsp) - ((1-fI)/2.86)*baob*naob*(Ko2aob/(Ko2aob+So2))*(Sno2+Sno3)/(Kno3aob+Sno2+Sno3)*Xaob - ((1-fI)/2.86)*bnb*nnb*(Ko2nb/(Ko2nb+So2))*(Sno2+Sno3)/(Kno3nb+Sno2+Sno3)*Xnb - ((1-fI)/2.86)*bnsp*nnsp*(Ko2nsp/(Ko2nsp+So2))*(Sno2+Sno3)/(Kno3nsp+Sno2+Sno3)*Xnsp - ((1-fI)/2.86)*bamx*namx*(Ko2amx/(Ko2amx+So2))*(Sno2+Sno3)/(Kno3amx+Sno2+Sno3)*Xamx-((1-fI)/2.86)*bhan*nhan*(Ko2han/(Ko2han+So2))*(Sno2+Sno3)/(Kno3han+Sno2+Sno3)*Xhan)
((Qo*Sno2o-Qo*Sno2)/V - (((1-YNo2han)/(1.71*YNo2han))*u6*nhan*(Sse/(KSsehan+Sse))*(Sno2/(Kno2han+Sno2))*(Ko2han/(Ko2han+So2))*Xhan) - (((u5/Yamx)+(u5/1.14))*(Snh4/(Knh4amx+Snh4))*(Sno2/(Kno2amx+Sno2))*(Ko2amx/(Ko2amx+So2)))*Xamx + ((u1/Yaob)*(So2/(Ko2aob+So2))*(Snh4/(Knh4aob+Snh4)))*Xaob - ((u2/Ynb)*(Sno2/(Kno2nb+Sno2))*(So2/(Ko2nb+So2))*Xnb) - ((u3/Ynsp)*(Sno2/(Kno2nsp+Sno2))*(So2/(Ko2nsp+So2)))*Xnsp)
((Qo*Snh4o-Qo*Snh4)/V - (u1*(INBM+1/Yaob)*(So2/(Ko2aob+So2))*(Snh4/(Knh4aob+Snh4)))*Xaob -(u5*(INBM+1/Yamx)*(Snh4/(Knh4amx+Snh4))*(Sno2/(Kno2amx+Sno2))*(Ko2amx/(Ko2amx+So2)))*Xamx - (u2*INBM*(Sno2/(Kno2nb+Sno2))*(So2/(Ko2nb+So2)))*Xnb -(u3*INBM*(Sno2/(Kno2nsp+Sno2))*(So2/(Ko2nsp+So2)))*Xnsp - INBM*u6*nhan*((Sse/(KSsehan+Sse))*(Sno2/(Kno2han+Sno2))*(Ko2han/(Ko2han+So2)))*Xhan - u6*INBM*nhan*((Sse/(KSsehan+Sse))*(Sno3/(Kno3han+Sno3))*(Ko2han/(Ko2han+So2)))*Xhan - u6*INBM*((Sse/(KSsehan+Sse))*(So2/(Ko2han+So2)))*Xhan + (INBM-(fI*INXI))*baob*naob*((Ko2aob/(Ko2aob+So2))*(Sno2+Sno3)/(Kno3aob+Sno2+Sno3))*Xaob + (INBM-(fI*INXI))*bnb*nnb*((Ko2nb/(Ko2nb+So2))*(Sno2+Sno3)/(Kno3nsp+Sno2+Sno3))*Xnb +(INBM-(fI*INXI))*bnsp*nnsp*((Ko2nsp/(Ko2nsp+So2))*(Sno2+Sno3)/(Kno3nsp+Sno2+Sno3))*Xnsp + (INBM-(fI*INXI))*bamx*namx*((Ko2amx/(Ko2amx+So2))*(Sno2+Sno3)/(Kno3amx+Sno2+Sno3))*Xamx + (INBM-(fI*INXI))*bhan*nhan*((Ko2han/(Ko2han+So2))*(Sno2+Sno3)/(Kno3han+Sno2+Sno3))*Xhan +(INBM-(fI*INXI))*baob*(So2/(Ko2aob+So2))*Xaob + (INBM-(fI*INXI))*bnb*(So2/(Ko2nb+So2))*Xnb +(INBM-(fI*INXI))*bnsp*(So2/(Ko2nsp+So2))*Xnsp + (INBM-(fI*INXI))*bamx*(So2/(Ko2amx+So2))*Xamx + (INBM-(fI*INXI))*bhan*(So2/(Ko2han+So2))*Xhan)
((Qo*So2o-Qo*So2)/V +KaL*(8-So2)-(((1-Yhaer)/Yhaer)*u6*(Sse/(KSsehan+Sse))*(So2/(Ko2han+So2)))*Xhan -(((3.43-Yaob)/Yaob)*u1*(So2/(Ko2aob+So2))*(Snh4/(Knh4aob+Snh4)))*Xaob - (((1.14-Ynb)/Ynb)*u2*(Sno2/(Kno2nb+Sno2))*(So2/(Ko2nb+So2)))*Xnb - (((1.14-Ynsp)/Ynsp)*u3*(Sno2/(Kno2nsp+Sno2))*(So2/(Ko2nsp+So2)))*Xnsp - (1-fI)*baob*(So2/(Ko2aob+So2))*Xaob - (1-fI)*bnb*(So2/(Ko2nb+So2))*Xnb -(1-fI)*bnsp*(So2/(Ko2nsp+So2))*Xnsp - (1-fI)*bamx*(So2/(Ko2amx+So2))*Xamx - (1-fI)*bhan*(So2/(Ko2han+So2))*Xhan)];
end

Accepted Answer

John D'Errico
John D'Errico on 9 Jul 2023
Have something like this be the last thing you do in the function.
x = max(x,0);
Or,
x(x<0) = 0;
  3 Comments
Steven Lord
Steven Lord on 1 Sep 2023
For this particular scenario, where you're trying to constrain the solution to a system of ODEs to be non-negative, rather than calling max at the end of the ODE function I'd use the NonNegative option for ODEs if it's supported for the ODE solver you're using. See this example in the documentation.

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