[Mesa-users] EOS: unphysical low-temperature μ from OPAL/SCVH and HELM?

[Farag, Ebraheem]

Hello Natasha,

Perhaps experimenting with the eos plotter in $MESA_DIR/eos/plotter. It wraps the eos and returns values for plotting or printing. It allows you to choose any eos option or blended.

-EbF
________________________________
From: Mesa-users <mesa-users-bounces at lists.mesastar.org> on behalf of Natasha Ivanova via Mesa-users <mesa-users at lists.mesastar.org>
Sent: Tuesday, May 12, 2026 7:37 PM
To: mesa-users <mesa-users at lists.mesastar.org>
Subject: [Mesa-users] EOS: unphysical low-temperature μ from OPAL/SCVH and HELM?

Hello all,


I wrote earlier about this, but have not received a response yet. After digging further, the issue looks more serious.

I am testing MESA EOS through an external wrapper. I scan the (ρ,T) plane and record the values returned by eosDT_get. The attached plots show the returned μ and logP.

At low temperatures, some returned values look clearly unphysical. For example, at

log rho = -13, log T = 3

MESA returns

mu = 0.6157

which corresponds to fully ionized solar-composition gas. At this density and temperature, neither thermal ionization nor pressure ionization should apply. The EOS fractions show that this point comes fully from OPAL/SCVH:

frac_HELM      = 0
frac_OPAL_SCVH = 1
frac_FreeEOS   = 0
frac_PC        = 0
frac_Skye      = 0
frac_CMS       = 0

There is also another low-temperature region, also circled in the attached plot, where the fractions show that HELM is responsible, and the returned μ is again physically implausible.

My questions are:

  1.  If OPAL/SCVH or HELM return clearly unphysical μ in these regions, should pressure and internal energy returned there be considered reliable?
  2.  Is res(i_mu) intended to represent the physical mean molecular weight corresponding to the ionization state, or can it be only an effective/composition-based value?
  3.  Is there a supported way, from an external wrapper, to call FreeEOS only, without fallback to OPAL/SCVH or HELM, so that I can test whether FreeEOS behaves correctly in these regions?

thank you,
Natasha




-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.mesastar.org/pipermail/mesa-users/attachments/20260513/e864e56d/attachment.htm>