I Rosenbrock method ODE : Abrupt increase during First iteration

gurbir_s
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I am using Rosenbrock4 method from Odeint library in C++ to solve a set of differential equations. However, I see that it gives an unusual change in the result for first iteration but afterwards, gives good results. How can I solve it?
The set of equations is
1682164625015.png

I have first tried to solve only first two equations (removing the components of other 4 equations from them.)

This is the output, where first column is the time, 2nd - X_p, and 3rd - X_n

1682164726299.png
 
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How are you setting your initial conditions for this solution?
 
berkeman said:
How are you setting your initial conditions for this solution?
The ##X_i##'s are elemental abundances during Big Bang Nucleosynthesis. I set the initial conditions equal to their thermal equilibrium values at that point.

I don't know how, but the problem got solved automatically while I was trying to solve another problem. Idk how. Thanks for your reply.
 

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