OVERVIEW OF MOLECULAR ORBITAL & MOLECULAR MECHANICS TECHNIQUES
Suggested Reading:
A Guide to Molecular Mechanics and Molecular Orbital Calculations, Hehre, et. al,
Overview p. 5-14; Comparison of Accuracy p. 19-78 (especially Conformational Energy Differences, p. 61-68)
Molecular Mechanics
Empirical Force Field
Speed of calculation: very fast--use for molecules of > 1000 atoms
Accurate for calculation of equilibrium geometry & conformational energy & differences in conformational energy
MMFF94 is better than SYBYL
Molecular Orbital Theory
Ab Initio: solution of Schrödinger Eq. using a wavefunction which is a sum of functions (orbitals, basis set functions) with variable parameters
Speed of calculation: very slow--use for molecules of < 100 atoms; Speed decreases significantly with increasing molecular size
Accuracy:
Excellent for equilibrium & transition state geometries
Reasonable for equilibrium conformational energies
Satisfactory for reaction thermodynamics & kinetics
Not accurate for:
Structures of transition metal inorganic & organometallic compounds
Reactions involving bond breaking & making (i.e. comparison of energy differences between reactants & transition states)
Ab Initio with Correlation Energy (MP2-Moller Plesset 2): Electron correlation correction is included in the energy
Speed of calculation: very slow--use for molecules of < 20 atoms
Accuracy:
Excellent for equilibrium & transition state geometries
Excellent for equilibrium conformational energies
Excellent for reaction thermodynamics & kinetics, including reactions where bonds are broken & formed
Semi-Empirical (AM1, PM3, etc.): solution of Schrödinger Eq. using a wavefunction which is a sum of functions (orbitals, basis set functions) with variable parameters--as in ab initio MO theory
Approximation: integrals set equal to zero or to parameters which are subsequently optimized by fitting to experimental data
Accuracy:
Excellent for equilibrium geometries
Reasonable for transition state geometries
Not accurate for:
Thermochemical & kinetic reaction evaluation
Conformational assignments
Speed of calculation: much faster than ab initio, less than molecular mechanics; use for molecules of up to 200 atoms