  We present an example of an interpolation code of the SAHA-S equation of
state that has been adapted for use in the stellar evolution code CESAM2k. The
aim is to provide the necessary data and numerical procedures for its
implementation in a stellar code. A technical problem is the discrepancy
between the sets of thermodynamic quantities provided by the SAHA-S equation of
state and those necessary in the CESAM2k computations. Moreover, the
independent variables in a practical equation of state (like SAHA-S) are
temperature and density, whereas for modelling calculations the variables
temperature and pressure are preferable. Specifically for the CESAM2k code,
some additional quantities and their derivatives must be provided. To provide
the bridge between the equation of state and stellar modelling, we prepare
auxiliary tables of the quantities that are demanded in CESAM2k. Then we use
cubic spline interpolation to provide both smoothness and a good approximation
of the necessary derivatives. Using the B-form of spline representation
provides us with an efficient algorithm for three-dimensional interpolation.
The table of B-spline coefficients provided can be directly used during stellar
model calculations together with the module of cubic spline interpolation. This
implementation of the SAHA-S equation of state in the CESAM2k stellar structure
and evolution code has been tested on a solar model evolved to the present. A
comparison with other equations of state is briefly discussed. The choice of a
regular net of mesh points for specific primary quantities in the SAHA-S
equation of state, together with accurate and consistently smooth tabulated
values, provides an effective algorithm of interpolation in modelling
calculations. The proposed module of interpolation procedures can be easily
adopted in other evolution codes.
