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Re: kinetic problem

Long time no talk. Hope everything is going right for you.

> My student Hui Tan and I have encountered the problem when using kinetics

with phreeqc. After reaching eqbm after about 0.1 year, it oscillates
between zero and negative rates --- meaning feldspar is precipitating. I
suspect that this is a numerical problem. Do you have any ideas how we can
fix or trick it?

Specify -tol 1e-12 in the KINETICS definition. This tolerance reduces the
oscillations to the 1e-15 range. By default PHREEQC integrates the
equations to a precision of 1e-8 moles. In this case, you need a smaller

> We have math profs here who are expert. If you describe
clear enough what is the problem, they may be able to help devise some
numerical solutions.

It would be advantageous to use an implicit method for integrating the rate
equations. I worked on this once with an implicit Runge-Kutta method, but
did not finish. This approach would help with stiff equations where the
rates differ by orders of magnitude, like when you want one reaction to be
nearly equilibrium and others slow. You may run into problems with stiff
equations with your set of rates, I'm not sure.


David Parkhurst (dlpark@xxxxxxxx)
U.S. Geological Survey
Box 25046, MS 413
Denver Federal Center
Denver, CO 80225

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