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Avoided crossings (Read 1617 times)
Gerrit-Jan Linker
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Avoided crossings
14.11.10 at 13:08:20
 
Avoided crossings
 
Avoided crossings are regions on the potential energy surface (PES) where a single reference description breaks down.
 
A classical example of an avoided crossing is that of a rotation over a double carbon bond. The ethylene molecule H2C=CH2 is a model system.
 
On rotating over the C=C double bond the two electrons in the out of plane pz functions overlap wel at the start (no rotation; or angle=0). When applying a rotation angle towards 90 degrees, the pz functions on the adjecent C atoms overlap less and less well and at 90 they are perpendicular and do not overlap at all. It is in this region where there is an avoided crossing.
 
To fix this problem the system can be described with multiple references. If the wavefunction is approximated by a linear combination bonding pi wavefunction and an anti-bonding pi* wavefunction then the region of 90 degrees torsion may be described much better.
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Gerrit-Jan Linker
Linker IT Software
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