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A frequently asked question is, "Which method in MOPAC is the most accurate?" Although apparently a simple question, there is no simple answer other than that PM7 and PM6-D3H4 are the two most accurate methods in MOPAC. These methods should be used for all routine use. There are several other methods, but they should only be used when necessary. The data in the PM7 and PM6-D3H4 Accuracy web-page provides an unbroken chain of logic from raw experimental data to a summary of a statistical analysis of the errors in various properties, and provides enough information to allow users of MOPAC to decide which method to use.
The methods in MOPAC were designed to provide practical chemists with a tool for modeling chemical systems. To this end, the focus has been on main-stream chemistry - think sulfuric acid - and on systems with common but interesting structures, for example hydrogen bonds in enzymes. Modeling methods are of little practical use unless they can be used without much effort, i.e., in a reasonable time on readily available hardware. These objectives can be contrasted with the philosophy behind many high-level methods, where the objective is to generate accurate data on small systems, or to model exotic systems or exotic phenomena. Such methods are often very time consuming. This is not an attempt to disparage such methods: if semiempirical methods are to be improved, they will have to depend on reference data generated using high level methods.
Advantages: The best method for general chemistry, and for solids. Optimized to reproduce the Standard Heat of Formation, ΔHf.
Disadvantages: Less accurate than PM6-D3H4 for geometries in organic chemistry. See crystal urea.
Advantages: Very good for biochemistry and organic and organometallic chemistry. Very good for non-covalent interactions. Errors in intermolecular interactions are much smaller than in PM7
Disadvantages: Often there are severe errors in inorganic solids. Errors in ΔHf are much larger than in PM7. This is a consequence of the fact that the D3H4 modification was added on to the PM6 method. PM6 was optimized to reproduce ΔHf, and the addition of D3H4 resulted in extra stabilization.
For all organic chemistry, use PM6-D3H4.
For inorganic solids, use PM7.
For more information, consult the statistics in the PM7 and PM6-D3H4 Accuracy web-page.