  New, high-resolution MARCS synthetic spectra have been calculated for more
than a dozen mixtures of the metals allowing, in turn, for variations in C:N:O,
[CNO/Fe], and enhanced abundances of C, O, Mg, and Si. Bolometric Corrections
(BCs) for many of the broad-band filters currently in use have been generated
from these spectra. Due to improved treatments of molecules that involve atoms
of C, N, and O, the BCs for UV and blue passbands, in particular, differ
substantially from those derived from previous MARCS models. These differences,
and the effects on BCs of varying the abundances of the metals, are shown in a
number of instructive plots. Stellar evolutionary grids for -2.5 <= [Fe/H] <=
-0.5 have also been computed for the different mixtures. Isochrones based on
these tracks are intercompared on the theoretical H-R diagram and on a few of
the colour-magnitude diagrams that can be constructed from HST Wide Field
Camera 3 (WFC3) F336W, F438W, F606W, F814W, F110W, and F160W observations. For
the first application of these models, isochrones have been fitted to WFC3
photometry of the globular cluster NGC 6496 from the HST UV Legacy Survey, with
very encouraging results.
