Accurate standard enthalpies of formation of 115 isolated pentagon rule (LPR) fullerenes with 60-180 carbon atoms have been derived from energies of isodesmic interconversion reactions computed at the B3LYP/6-31G* level of theory. The calculated values of Delta H(f)degrees which may serve as benchmarks for both calorimetric measurements and less sophisticated theoretical studies, are reproduced within 3 kcal/mol by a simple scheme based upon counts of 30 distinct structural motifs composed of hexagons together with their first and second neighborhoods. The extremely low computational cost of such a scheme makes it ideally suited for a rapid prescreening for thermodynamically viable IPR fullerenes with cages composed of hundreds of carbon atoms. With the inclusion of a global curvature term, this scheme is expected to be equally successful for small and large carbon clusters.