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Selecting a Model
A brief overview of the performance of molecular mechanics and quantum chemical
models (including semi-empirical models, Hartree-Fock models, density functional
models and MP2 models) with regard to the calculation of equilibrium
and transition-state geometries, conformations and reaction thermochemistry is
provided below. This is based on an extensive series or comparisons presented in
For each task the methods are "graded" as follows
## In Terms of Task:Geometries:Hartree-Fock models with basis sets larger than 6-31G* do not provide significantly improved descriptions of either equilibrium or transition-state geometries over the Hartree-Fock 6-31G* model and, in most cases, the 3-21G model. Note, however, that MP2 and density functional models require (at the very least) basis sets which incorporate polarization functions. Hartree-Fock models do not provide a reliable account of the geometries of compounds incorporating transition metals, but the PM3 semi-empirical model and density functional models provide good accounts. MP2 models provide good geometries for some transition metal systems, but very poor geometries for others. Conformational EnergiesSemi-empirical models may in some cases be suitable for identifying conformational minima, and for determining the geometries of these minima, but they are not suitable at providing accurate relative conformer energies. The MMFF molecular mechanics model provides an excellent account of conformational energy differences in organic compounds. Note, however, that most of the data used in the assessment were also used to parameterize MMFF. Caution is needed in the application of MMFF outside the original range of its parameterization. EnergeticsMP2 and density functional models are needed to accurately account for the energetics of reactions where bonds are broken or formed and to properly describe absolute activation energies. Hartree-Fock and local density models are unsatisfactory. Hartree-Fock models are satisfactory in description of relative activation energies. Hartree-Fock and MP2 models provide an excellent account of the energetics of isodesmic reactions. Density functional models are not reliable in all instances and should be applied only with caution. Semi-empirical models are unsatisfactory in their description of the energetics of all types of reactions, isodesmic processes included. ## In Terms of Method:
Semi-empirical models are particularly attractive for:
- Equilibrium geometry determinations for large molecules, where the cost of Hartree-Fock and MP2 and density functional models may be prohibitive.
- Transition-state geometry optimizations, where the cost of Hartree-Fock and MP2 and density functional models may be prohibitive.
- Equilibrium and transition-state geometry optimizations involving transition-metal inorganic and organometallic compounds, where Hartree-Fock models are known to produce poor results, and where the cost of MP2 and density functional models may be prohibitive.
Semi-empirical models are unsuitable for:
- Calculations on reaction energies, including the energies of isodesmic processes.
- Calculations of conformational energy differences.
Hartree-Fock models are particularly attractive for:
- Equilibrium and transition-state structure determinations of medium-size organic and main-group inorganic molecules, where increased accuracy over that available from semi-empirical models is required, and where the cost of MP2 and density functional models may be prohibitive.
- Calculations of reaction energies (except reactions involving explicit bond making or breaking), where semi-empirical models yield unacceptable results, and where the cost of MP2 and density functional models may be prohibitive.
Hartree-Fock models are unsuitable for:
- Calculation of reaction energies which involve net bond making or breaking and calculation of absolute activation energies.
- Equilibrium and transition-state structure determinations for transition-metal inorganic and organometallic molecules.
- Calculations on large molecules, where the cost of and MP2 models may be prohibitive, and where Hartree-Fock semi-empirical calculations may not be sufficiently accurate. li>Calculations on inorganic and organometallic systems where Hartree-Fock models may not be sufficiently accurate, and where the cost of MP2 models may be prohibitive.
- Thermochemical calculations, in particular, those which involve explicit bond making or breaking, and absolute activation energy calculations.
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