Here is an input file to calculate the structure of water - it performs a geometry optimization, to find a minimum energy structure. 

 

The geometry is given as a Z-matrix (the geometry is the same as the Cartesian coordinate example above). 

 

# RHF/STO-3G OPT  ¬Keywords (OPT specifies a geometry optimization)

 

Geometry optimization of water  ¬(Title)

 

0 1    ¬Charge and multiplicity of water

O                   }¬Z-matrix

H 1 0.96           }¬Z-matrix

H 1 0.96 2 109.471    }¬Z-matrix

 

 

This geometry is just a model produced by a molecular building program. 

 

Its structure is optimized here using the electronic structure program Gaussian-98. 

 

This program uses molecular orbital methods to calculate the energy of the molecule, and the forces on the nuclei, at each step, and optimizes the geometry.

 

In this case, it takes three steps to reach a minimum. 

 

This should give a structure which is an energy minimum for the level of theory used. 

 

Next: progress of the minimization