Geometry Optimizations
- the process of determining the energy minima
- adjustment of bond angles and lengths that give the lowest possible energy value
If the bond length and angle of water are thought of as variables along the x and y axes and energy of that configuration is displayed along the z axis (height), then the three-dimensional surface which is created is called the "potential energy surface" for water. A portion of that surface is displayed below. A geometry optimization "walks" along this surface by making small changes in the geometry coordinates until it reaches (hopefully!) a basin.
- an iterative process that starts with a guessed geometry and ends up with an optimized geometry
- A successful optimization only guarantees a local minima, not a global minima. Repeated optimizations with other structures (conformers) must be conducted
Strategies
- use a semi-empirical level of theory (AM1) FIRST then re-optimize with ab initio (3-21G or 6-31G)
- one feature of a geometry is the symmetry of the molecule. If this property interferes with a successful optimization, turn it off
- nosymtry keyword (in option box)
- sometimes an optimization requires more cycles (iterations) than provided (250). If more are needed
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