Barrier heights can be calculated with increased accuracy by using PM7-TS. The PM7-TS method was parameterized to reproduce high-level ab-initio barrier heights, given a PM7 reaction geometry and a transition state geometry. (The reaction geometry is the geometry of the system before ascending the activation barrier.)
For PM7-TS to work, the reaction geometry must be first calculated using PM7. The transition state must also be located and refined using PM7. To use PM7-TS, run one calculation using the reaction geometry, and one calculation using the transition state geometry, then the barrier height is obtained by subtracting the heat of formation of the reaction geometry system from that of the transition state geometry system.