The "piecewise-Gibbs" species thermodynamic model, implemented by class Mu0Poly is described as implementing "an interpolation of the Gibbs free energy based on a piecewise constant heat capacity approximation".
If you fit a Mu0Poly object using a set of points for a species, using a different model (e.g. NASA polynomials) to evaluate the standard Gibbs free energy at the interpolation point:
you can see that it does indeed match the specified values of $g^\circ$ at the interpolation points (green circles). However, the interpolated values (orange) oscillate more than you might expect for a fit of a function that's practically a straight line. Things get really wild when you plot the heat capacity, enthalpy, and entropy values:
While the heat capacity is a constant in each interpolation region as specified by the model, the values are bonkers, oscillating between positive and negative values, for values that are about 2 orders of magnitude higher than they should be. This in turn results in the zig-zag behavior of the enthalpy and entropy.
Problem description
The "piecewise-Gibbs" species thermodynamic model, implemented by class
Mu0Poly
is described as implementing "an interpolation of the Gibbs free energy based on a piecewise constant heat capacity approximation".If you fit a
Mu0Poly
object using a set of points for a species, using a different model (e.g. NASA polynomials) to evaluate the standard Gibbs free energy at the interpolation point:you can see that it does indeed match the specified values of $g^\circ$ at the interpolation points (green circles). However, the interpolated values (orange) oscillate more than you might expect for a fit of a function that's practically a straight line. Things get really wild when you plot the heat capacity, enthalpy, and entropy values:
While the heat capacity is a constant in each interpolation region as specified by the model, the values are bonkers, oscillating between positive and negative values, for values that are about 2 orders of magnitude higher than they should be. This in turn results in the zig-zag behavior of the enthalpy and entropy.
Steps to reproduce
System information
main
branch at 3a9bc6a66