Semi-empirical methods

• Names for some commonly used semi-empirical methods are (in chronological order of development):

 

CNDO	crude, fast, can do second row elements
INDO	better for first row elements
MINDO3
MNDO
AM1	better for H bonds
PM3	developed from AM1;  includes more main-group elements

• For ordinary molecules, AM1 or PM3 are probably the best to try
• Semi-empirical methods are most useful for:
• better geometry optimisation than force fields
• geometries for molecules including atoms which are not parameterised in a force field
• qualitative prediction of IR frequencies
• total electron density surface for graphical display
• They are not really good enough for reaction energies and equilibrium constants
• Even quite low level ab initio methods are better for energetics
• They cannot do anything to do with core electrons, e.g. NMR shieldings
• The ZINDO method can deal with excited states, which are more difficult to do than ground states in ab initio methods.  Hence predictions of UV/visible spectrum absorption wavelengths are possible.