Optimization of parameters for semiempirical methods V: modification of NDDO approximations and application to 70...

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Method Development

Background:

• The Neglect of Diatomic Differential Overlap (NDDO) method is a semi-empirical quantum chemistry approach.
• Parameter optimization is crucial for the accuracy of NDDO methods.
• Existing methods like AM1 and PM3 have known limitations and inaccuracies.

Purpose of the Study:

• To introduce and validate a new, improved parameterization for NDDO methods, termed PM6.
• To expand the elemental coverage of NDDO parameterizations.
• To enhance the accuracy of calculated heats of formation and molecular geometries.

Main Methods:

• Modifications to the NDDO core-core interaction term.
• Development of an advanced parameter optimization strategy.
• Parameterization of 70 elements for the PM6 method.
• Validation against experimental heats of formation for 4,492 species.

Main Results:

• The PM6 method achieved an average unsigned error (AUE) of 8.0 kcal mol(-1) for 4,492 species.
• For a subset of 1,373 common elements, PM6 yielded an AUE of 4.4 kcal mol(-1), outperforming other methods.
• PM6 demonstrated superior performance compared to RM1, B3LYP 6-31G*, PM5, PM3, HF 6-31G*, and AM1.
• Long-standing errors in AM1 and PM3 were corrected, and geometry predictions were significantly improved.

Conclusions:

• PM6 represents a substantial advancement in semi-empirical quantum chemistry.
• The method offers improved accuracy and broader elemental applicability for molecular modeling.
• PM6 is a valuable tool for predicting thermochemical properties and molecular structures with higher fidelity.