Hello all: I'm using a code for solving partial differential equations that describe transient advection and diffusion. I'm more of a chemistry person, so I'm a bit lost on some of the basics of the calculation method. A general question: As my calculation progresses, I see that some of the A coefficient values (in the equation Ax = b) go negative. This seems weird because all of the values used in the problem represent measurements that I carried out in my lab (positive). So I'm trying to figure out whether there is some mathematical reason (like error? Something else I haven't thought of?) why these negative values may be arising or whether there is a bug in the code. I'm not sure if I have provided enough information for anyone to speculate, but I'd be interested in hearing any suggestions. Thanks a lot!