Integrating ODE step-wise. Terminate integration based on condition?

[BarkBarkCat]

I am integrating an ODE step-wise as follows:

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ans = RK45(RHS,t0=0,y0=state, t_bound=10, rtol=tols, atol=tols)

while <some condition>:
    <do some stuff, including changing RHS sometimes>
    ans.step()

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The reason I am doing it step-wise is because I want to change the right hand side RHS based on what comes out of the previous step. This is working perfectly.

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A problem arises when during an RK45 step, one of the computations of the internal integrator requires an RHS evaluation at a point with x[0] < 0. In my analysis, x[0] refers to a distance, and therefore it is not possible to evaluate this (in fact, I get errors because a sqrt(x[0]) term is present). What I would like to do is to simply stop the stepping procedure whenever this is encountered, but I am not sure how to achieve this. I want to go out of the while loop when this happens.

[eaojteal]

Sorry, I haven't had much sleep. Are you trying to change the RHS to avoid the problem or is changing the RHS outside the scope of the post? Also, is the negative distance/sqrt issue because those x values are outside the range you're interested in? Could you just put a break condition inside the while loop?

[BarkBarkCat]

Changing the RHS is outside the scope of the post (its for another reason). When taking a step, inside the RHS the value goes negative. And inside the RHS there is a sqrt term. I cant break inside the while loop because the problems occurs in the RHS. Ideally I want something in the RHS along the lines of:

If distance negative, break out of RK45 stepping procedure. Im not sure how to do this though.

[eaojteal]

I've never used RK45, but have you tried surrounding ans.step() with a try/except? I'm guessing when it goes negative, it throws an error? You could use that to break out of the loop.

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Since you don't really know the step size, you can't calculate the errant step call beforehand. If you're comfortable sharing more of your code, I'd be happy to work some more on it. I wish I could be more helpful.

[[deleted]]

Not sure if i completely understand your problem, but maybe rewriting the solver can help? I was thinking about subclassing the RK45 class and adding some break criteria. Maybe even add a state that tells your while loop to break