Recommendations for Selecting Research Models

The following table summarizes some recommended research-level model chemistries for predicting total energies for molecular systems of various sizes (the details of the reasons behind these choices will unfold along with the chapters in this part of the book), arranged in order of increasing computational cost  

Model Chemistry Range of MAD /Max. Error Applicability on G2 Molecule Set [ heavy atoms] Relative Cost 

Larger, floppy molecules might need to be optimized with a smaller basis set. 

The general strategy embodied by this table is to select the most accurate calculation that is computationally practical for a given size system. Note that for the lower-cost methods, you will also need to add diffuse functions and/or additional polarization functions on the hydrogen atoms as appropriate for the systems you are studying. 

Selecting Methods (or Zero-Point and Thermal Energies 

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