How semi empirical method is different from empirical method. Is there any guidelines/ or order of preference to use one of the method, i mean how to find the suitability of one method over other?
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
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
Dear Sivakumar Sir,
Thanks a lot for your time and effort. Actually I am not exposed to/ familiar with Hartree-Fork method and all. As a layman (in theoretical/ computational chemistry) what I understood from your explanation is semi empirical method needs experimental data as well as some approximations in contradiction to empirical method where only experimental data is enough. Is that what you are trying to say.
Hi Pratab
Semi-empirical method is a cheap method because the calculation is done using experimental data to get the results. For example, Hartree Fock (HF) method is purely theoretical, hence, HF calculate the bond distance and further properties. On the other hand, semi-empirical method has experimental information preloaded about bond distance and other properties, therefore it do not need to calculate them. For that rason, the semi-empirical method is very fast that you can calculate organic molecules bigger in little time. The experimental information has been preloaded by the performer of the method, hence, you do not need to have the experimental data for the using the semi-empirical method. I think the PM7 method is the best and you can use it with MOPAC software.
Best Regards.
Semi Empirical metods are considered as reference for experiential mock ups in our engineering models.
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