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 tolerance. > 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 David Parkhurst (dlpark@xxxxxxxx) U.S. Geological Survey Box 25046, MS 413 Denver Federal Center Denver, CO 80225 Project web page: http://wwwbrr.cr.usgs.gov/projects/GWC_coupled
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