If a job runs to completion, but the calculated Heat of Formation, ΔHf, is very large (over 10,000 kcal/mol) is likely that there is a fault in the geometry.  If the job is re-run using keyword CHECK, the interatomic distances will be checked for atom distances of less than 0.9 Å.  If there are any such short distances, they will be reported and the job stopped.  If none are found, the run will continue.