I have been working on a method for integrating the rate equations that works better with stiff equations. I have implemented a package called CVODE from Lawrence Livermore National Lab as an alternative to the Runge-Kutta method that should speed up the calculations. This version should work better when you are trying to simulate equilibrium by using fast rates, which is the situation in the problem you sent me earlier. You should be able to use the enclosed version without having to increase the rate constants to be on the order of the time step. I have done some preliminary testing and the program seems to work, but you should do some additional testing to make sure it is working correctly on your problems. The enclosed program has identical functionality to PHREEQC, except this version uses CVODE to integrate rate expressions, which could be slower than RK in some (many?) cases. Please let me know if you find any bugs and, if you use it, whether it works satisfactorily. David (See attached file: phreeqc.cvode.exe) David Parkhurst (dlpark@xxxxxxxx) U.S. Geological Survey Box 25046, MS 413 Denver Federal Center Denver, CO 80225 Project web page: https://wwwbrr.cr.usgs.gov/projects/GWC_coupled
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