TUM Quantum Chemistry Laboratory
Prof. N. Rösch

Exercise 4

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Construct a Z-Matrix for methane, CH4 (Td). Use the experimental C-H distance, 1.085 Å [1], and the tetrahedral angle, about 109.47122. This value is precise enough so that the program Gaussian will replace it by the exact value. If the value is less accurate, Gaussian will not recognize the tetrahedral angle and will thus break the tetrahedral symmetry.

As in Exercise 2 the Z-matrix has to preserve the symmetry of the molecule during geometry optimization.

The high tetrahedral symmetry of the methane molecule may pose a challenge for the novice. Therefore the first level of solution to this exercise contains several hints for setting up the Z-matrix.


Z-Matrix

Atom Distance to atom Angle Dihedral angle
N1 symbol N2 N1-N2 special
option
N3 N1-N2-N3 special
option
N4 N1-N2-N3-N4 special
option
no. no. Å no. degree no. degree
1
2
3
4
5

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

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Send in

To have your solution checked, please send in the Z-matrix (Gaussian input) for CH4 restricted to tetrahedral symmetry Td .

Reference

[1] D. L. Gray and A. G. Robiette, "The anharmonic force field and equilibrium structure of methane", Mol. Phys. 37 (1979) 1901.


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Copyright © 2003 N. Rösch, Technische Universität München