Preparing Input for Geometry Optimizations

The Opt keyword in the route section requests a geometry optimization, using the basis set and level of theory specified by the other keywords. 

In Gaussian, the molecule specification for a geometry optimization can be given in any format desired Cartesian coordinates, Z-matrix, mixed coordinates. The geometry optimization job will produce the optimized structure of the system as its output. 

Previously, the requirements for molecule specifications for geometry optimizations were more stringent, and a large part of learning to perform geometry optimizations consisted of learning how to set them up properly. However, recent research into alternative coordinate systems and optimization procedures has made aU of this unnecessary. This topic is considered in Exercise 3.8 (page 57) see the references for further information. 

Ethylene is a highly symmetric molecule. Here is the input file for an optimization ol its geometry  

The values of 180° for all three dihedral angles specify the molecule in a planar orientation. 

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