Comparison of semiempirical MO methods applied to large molecules

The heats of formation for 19 molecules have been calculated with PM3 and AM1 semiempirical methods. The values obtained have been compared with experimental heats of formation. With PM3 and AM1 the average differences between calculated and experimental heats of formation are 8.45 and 12.34 kcal mol^?1 respectively. There are significant differences when large molecules are considered: this suggests that the parameterization should be done including larger molecules.