I think this is an indexing error for fixed mixing ratio upper boundary conditions https://github.com/VirtualPlanetaryLaboratory/atmos/blob/cdfe1b0bc30dc396cb946dbf7079643bbd2236d9/PhotoMain.f#L2020 I think the `K` should be replaced with an `I`. The code should be ```fortran do M=1,NQ MM = KD + I - M DJAC(MM,M) = 0. enddo ```