# [mesa-users] Proper use of eosDE

Philip Chang chang65 at uwm.edu
Sun Jul 23 02:04:52 EDT 2017

```Hi,

I'm integrating the mesa eos module into a hydro code that I am
writing/written.  I have lots of experience with Frank Timmes Helmholtz
eos (which I got from his website), but for my new code I am going to
use the MESA eos.  Anyhow, my hydro code gives me an internal energy and
density and I want to get other thermal quantities, i.e., temperature,
entropy, adiabatic index, etc.

The way to do this is using eosDE_get, but I'm not sure what the proper
use of this was and was hoping that someone knew offhand. I am using the
latest public release 9793.  In particular, my questions are:

1. How does one decide on a proper T_guess?  I looked in the mesa-star
code, and it appears that one want to use the temperature from the
previous step.  I'm wondering how good is this as a shock can move
though a cell and totally mess everything up.

2. An improper T_guess can usually be corrected, i.e., it produced
values that are obviously wrong.  I have tried a guess of 1e2 and 1e8
and usually if one fails then the other gives you a good value. But this
is not always the case, for instance, consider this particular
pathological case: rho = 794.32840915212967, T=158489.31924611141, solar
metallicity.

For T_guess = 1e7, I find T = 202665.40667059086

For T_guess = 1e2, I find T = 257198.81215480756

In this case both are wrong by order unity.  This is a particular bad
case as both guess give poor answers.  But there are cases where one
guess give a good answer, but the other does not, and it is not obvious
which one I should trust.  Any ideas?

3. What happens when the internal energy is too low?  For instance, in
my previous experience with the Helmholtz equation of state (from Frank
Timmes website), small fluctuations can drop the internal energy below
the fermi energy.  When this energy is use to try to find a temperature
in the Helmholtz equation of state, the code essentially crashed (as it
should).  In this case, I prechecked the internal energy to ensure that
it is valid.  What happens if I give the eosDE_get an internal energy
that is physically too small.

Thanks in advance for any help.

Cheers,

Phil Chang

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