Semi-Empirical Molecular Orbital Methods

Question 1

Semiempirical MO Methods

Answer 1

Approximations of HF-SCF that make it faster without making it less accurate
Often involve adapting a parametric form for some aspect of the calculation, and the parameter values chosen to best reproduce experimental data

Question 2

When are semiempirical MO methods used today?

Answer 2

For very large organic or organometallic molecules (greater than 40 non H atoms)
Unlike HF, semiempirical methods can be used for transition state optimization

Question 3

CNDO Methods

Answer 3

(Complete neglect of differential overlap)
- core electrons ignored
- valence electron LCAO-MO basis sets have one STO per AO
(STO = slater type orbital)

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Question 4

How do CNDO models differ?

Answer 4

Differ by their choice of parameter values
Poor accuracy but does not depend as much on the ‘training set’ (like MM methods do) because the equations are based in QM not classical mechanics

Question 5

INDO Methods

Answer 5

(Intermediate neglect of differential overlap)
- just like CNDO’s except for the ERI’s
- all one atom ERI’s are considered and given individual parameters

Question 6

NDDO Methods

Answer 6

(Neglect of diatomic differential overlap)
- just like INDO’s, except for the ERI’s
- all two atom ERI’s included

Question 7

Famous NDDO models

Answer 7

MNDO
AM1
PM3

Question 8

Summary

Answer 8

Semiempirical methods are ‘an approximation of an approximation’
Due to approximation, semi’s can do better than the original approximation (HF-SCF)