This formulation for the reactor's governing equations significantly improves
the performance of integrator, mostly by improving the quality of the
Jacobian. The effect is small for smaller mechanisms (GRI 3.0) but can lead to
order-of-magnitude improvements for mechanisms with hundreds or thousands of
species.
Since this set of variables corresponds to the intrinsic state variables used
for IdealGasPhase objects, we also eliminate the need to iterate when setting
the state of the thermo object.
Additionally, by using temperature as an independent variable, the
temperature-dependent parts of the kinetic rate expressions do not need to be
recomputed while updating the Jacobian (this optimization is not currently
implemented).
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