Electron Correlation - Configuration InteractionBecause electrons are negatively charged, electrons like to avoid each other. When one electronmoves the positions of the other electrons in the molecule adjust to minimize electron-electronrepulsion. Therefore, the motions of the electrons are not independent. In other words, the motionof the electrons is correlated. The current best method for adjusting the molecular wavefunctionsfor electron correlation is called configuration interaction, or CI for short. CI calculations allowthe ground state of a molecule to mix with excited states of the molecule. For example for the H2molecule, the ground state configuration and several excited states are diagrammed below.σσ*HH1s1sEσσ*1s1sEσσ*1s1sEGround StateSingle ExcitationDouble ExcitationIn CI calculations the ground state wavefunction is allowed to be a combination of all of thepossible excited states:Egs=0.9987*σσ*1s1sE+ 0.0518* σσ*1s1sE
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