[Mesa-users] Clarification about extra heat arrays in `run_star_extras`

[Ian Bania '21]

Hi all,

I have been working on a run_star_extras routine which makes use of the
other_energy_implicit hook. I have a pretty good grasp on the extra_heat
array itself, but I was wondering if anyone might be able to provide some
insight on, or point me to some documentation about these arrays:
          d_extra_heat_dlndm1, d_extra_heat_dlnd00, d_extra_heat_dlndp1, &
          d_extra_heat_dlnTm1, d_extra_heat_dlnT00, d_extra_heat_dlnTp1, &
          d_extra_heat_dlnR00, d_extra_heat_dlnRp1.
My current understanding is that these are partial derivatives of the
extra_heat array, with lnd** being with respect to the log of the density,
lnT** the log of the temperature, and lnR** the log of the radius. I'm
mostly confused about what is notated by the *00, *p1, and *m1 suffixes.
Looking through star_data/public/star_data.inc it appears that *00 just
refers to a normal partial, whereas *p1 and *m1 refer to the same partial
derivative but the variable we are differentiating with respect to is
offset up or down a cell? Is this the correct interpretation, and if so
does anyone have any advice for the best way to go about actually computing
these offset partials?

Thanks for any help!
Best,
Ian K. Bania
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