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What does the error message; N-R didn't converge after 999 its., maximum residual = 1, Xi = 0.0000 mean


hayleycas
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When I am trying to run simulations of water mixing, reaction of groundwater and mineral phase (among others) I keep getting this error message:

 

N-R didn't converge after 999 its., maximum residual = 1, Xi = 0.0000

 

I have no idea what it means and as such am not able to work out what to do with my data to enable my simulations to work.

 

I would appreciate some feedback explaining this error message, as it is completely baffling me and I cannot seemed to find the answer anywhere in the manuals.

 

regards,

Hayley

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Hi Hayley,

 

This typically means that the software is having trouble solving for the equilibrium state of the system. It could be because the system you are trying to model is out of equilibrium and so a numerical solutions for the equilibrium state cannot be found. Alternatively, you may be entering input in a way the program does not expect, or which does not lead to an easy solution.

 

A few tips. Try to start with a simple model, then add complexity as you go. Simply throwing everything you have at the GWB is generally not the best way to go. When you are constraining your system, make sure the species that you have loaded (or swapped) into the basis make sense. For example, at very low pH, you'll want to swap in CO2(aq) for HCO3-, and at high pH, swap in CO3-- for HCO3-. Similarly, choose basis species that match up with the redox state of your system. Sometimes, you'll want to decouple redox pairs which may be out of equilibrium in natural waters.

 

You might also experiment with the parameters that the program uses to find a numerical solution (Configure menu). Epsilon, the convergence criterion, can be adjusted to change how closely solutions in subsequent iterations must match. In reaction path simulations, you might reduce delxi if your solution appears to be numerically unstable. It is probably best to experiment with modifying your input though, rather than messing around with these parameters, until you have a good understanding of what they do.

 

Try messing around with a simplified system that works and then add more complexity. Spend some time with this and then a script to the forum so we can work on specifics.

 

Hope this helps,

Brian Farrell

Aqueous Solutions LLC

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