Failing to blend EOS around Z=0.04

[noahjdowning]

When diffusion is turned on and Z drops below 0.04, the EOS changes. This leads to discontinuities in the star's evolution.

This was found while running a ~0.65 Msun model at Z a little larger than 0.04 with do_element_diffusion = .true.

Setting OPAL_SCVH = .false. removes this discontinuity.

Below is surf_cell_z vs. logTeff for model grid of a ~0.65 Msun star on the main sequence. Note that when surf_cell_z crosses 0.04, there is a sharp change in logTeff.

Version

• MESA version [r24.08.1]
• MESA SDK VERSION [aarch64-macos-25.8.1.pkg)]
• OS: [MacOS 15.6.1]