Dear Pratap, Empirical evidences are the outcome of an experiment and hence the empirical result is a unified confirmation. But the, semi-empirical methods uses part of experimental results; approximations... Applying full Hartree–Fock method without any approximations is expensive due its more computing time. In applying these approximation the guidelines given in the Gaussian software will be useful. Semiempirical Methods such as AM1, PM3, MNDO semiempirical methods replace as many calculations as possible with pre-calculated data that are an integral part of the program. This speeds up calculation to such an extent that even large molecules can be calculated. For organic molecules, accurate structures can be calculated within minutes with the AM1 method (M. J. S. Dewar, E. G. Zoebisch, E. F. Healy, J. J. P. Stewart, J. Am. Chem. Soc. 1985, 107, 3902-3909. The PM3 method is more accurate than the AM1 method and is also parametrized for many transition metals. An even further improvement has been its successor the PM5 method. It gives up to four times more accurate heats of formation than the other semi empirical methods AM1, PM3, and MNDO. Many inorganic molecules can be calculated with the MNDO method