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Electron Correlation - Configuration Interaction
Because electrons are negatively charged, electrons like to avoid each other. When one electron
moves the positions of the other electrons in the molecule adjust to minimize electron-electron
repulsion. Therefore, the motions of the electrons are not independent. In other words, the motion
of the electrons is correlated. The current best method for adjusting the molecular wavefunctions
for electron correlation is called configuration interaction, or CI for short. CI calculations allow
the ground state of a molecule to mix with excited states of the molecule. For example for the H
2
molecule, the ground state configuration and several excited states are diagrammed below.
σ
σ
*
HH
1s
1s
E
σ
σ
*
1s
1s
E
σ
σ
*
1s
1s
E
Ground State
Single Excitation
Double Excitation
In CI calculations the ground state wavefunction is allowed to be a combination of all of the
possible excited states:
E
gs
=
0.9987*
σ
σ
*
1s
1s
E
+ 0.0518*
σ
σ
*
1s
1s
E

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