|Quantum Chemistry Laboratory|
|Prof. N. Rösch|
Dummy atoms are auxiliary points in three-dimensional space which facilitate the definition of the proper structural parameters and thus the construction of a Z-matrix. Dummy atoms neither feature a charged nucleus nor a basis set. They are fully ignored in the determination of the molecule's symmetry and in quantum chemical calculations. Dummy atoms are invoked by the symbol "X".
In this exercise a dummy atom shall be used to avoid bond angles
of 180 degrees in the Z-matrix of carbon dioxide.
Since that molecule is linear the O-C-O angle has 180 degrees.
For geometry optimizations in user-specified internal coordinates
Gaussian94 does not accept angles of 180 and 0 degrees in the
With the help of a dummy atom it is easy to devide the O-C-O
angle into two parts of less than 180°. Just use the angles
X-C-O1 and X-C-O2 in the Z-matrix.
The figure below shows a possible solution with a purple
colored dummy atom.
The number of free parameters to be calculated in an geometry optimization must not enlarge by using a dummy atom! Otherwise the geometry optimization would not converge. So fix a suitable number of structure parameters or define common variables.
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