The complete set of 187 isolated-pentagon-rule (IPR) isomers of C-96 has been systematically investigated by full geometry optimizations with various quantum chemical semiempirical methods as well as molecular mechanics, and their energetics are also computed at the ab initio HF/4-31G level. Some lower-energy isomers are further optimized at the B3LYP/6-31 G level. All of the applied methods point out a D-2 species as the system ground state. Since the energetics alone cannot predict relative stabilities at elevated temperatures, entropy contributions are also taken into account, and the relative-stability problem is entirely treated in terms of the Gibbs function. Considerable temperature effects on the stability interchanges in the isomeric set are found, and good agreement of the computations with available observed data is achieved.