TUM Quantum Chemistry Laboratory
Prof. N. Rösch

Exercise 2

Click here to load into WebLab Topic: Z-matrix with special options

The aim of a geometry optimization is to find a stationary point, often a local minimum but also a saddle point (transition state) on the energy hypersurface. Atom-atom distances, bond angles, and dihedral angles are changed by the quantum chemistry program until a stationary point is reached or, more precisely, the stopping criterium is fulfilled.

In this exercise a Z-matrix for formaldehyde shall be constructed which enforces preservation of symmetry during geometry optimization. The Z-matrix of Exercise 1 is not able to do that because each structural parameter is varied independently. It is not sufficient that the symmetry conditions are fulfilled at the beginning of the optimization process since rounding errors may destroy symmetry later on.

To preserve C2v symmetry

Use special options in the Z-matrix form to do that. In the form below you only have to fill out the last row. The first three rows contain the experimental values already used in Exercise 1.

As in that exercise, you may correct your entries if you discover a mistake. The solution obtained below is one of several correct alternatives.


Atom Distance to atom Angle Dihedral angle
N1 symbol N2 N1-N2 special
N3 N1-N2-N3 special
N4 N1-N2-N3-N4 special
no. no. Å no. degree no. degree

Allowed special options are

Do not use more than one option per parameter. A geometry parameter re-using a fixed value will be fixed automatically.

Z-Matrix as Program Input

(WebLab Viewer must be installed)

Send in

To have your solution checked, please send in the Z-matrix (Gaussian input) of H2CO restricted to symmetry C2v .


[1] K. Takagi and T. Oka, "Millimeter wave spectrum of formaldehyde", J. Phys. Soc. Japan, 18 (1963) 1174.

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