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.