Reactive transport codes are today one of the cornerstones of
environmental research. They now contain multiphysics with very complex
algorithms, including flow, transport, chemical and sometimes heat
transport, mechanical and/or biological algorithms. Because of this
complexity, some parts of these algorithms still have not been
sufficiently studied. Here, we present a comparison of 3 algorithms for
activity correction, a specific subset of equilibrium chemistry
algorithms. We show that the most used algorithm (the inner fixed-point
algorithm) or the most rigorous algorithm (the full Newton) might not be
the most efficient, and we propose the outer fixed-point algorithm,
which is more robust and faster than other algorithms.