Making a Data File for a Symmetric Top Molecule

A simple symmetric top molecule file can be generated from File, New, Symmetric Top This initializes the program for a parallel band of a molecule of C_3v symmetry with a band origin of 1500 cm^-1and with rotational constants in both states of A = 5 cm^-1, B = 0.5 cm^-1 . To adjust the constants (and set the type of vibronic transition) use select View, Constants. Most constants and settings can be left unchanged at their default values (and indeed should be unless you need a specific effect) but the ones that must normally be changed are adjusted as follows:

At the Molecule level (Click on "SymmetricTop") the PointGroup and the statistical weights are the most important. A symmetric top must have equivalent nuclei and the defaults of 1 are only appropriate for 3 equivalent spin ½ nuclei in C_3v. Note that you should leave nNuclei at 0 unless you specifically want to calculate hyperfine structure.

Adjust the ground state by clicking on the "v=0" entry under "Ground". The key things to set are

The Symmetry of the state
The rotational constants, A/C and B. Note that the rotational constant about the symmetry axis is always shown as C, though this corresponds to A for a prolate top.

Adjust the upper state by clicking on the "v=1" entry under "Excited". The key things to set here are:

The Origin
The Symmetry and rotational constants, as for the ground state.

The transition moment "<v=1|a|v=0>" may also need adjusting, as the default is along the a axis. The axis is likely to change if you change the symmetry and for low symmetry asymmetric tops more than one transition moment may be needed. (Right click on the transition moment in the tree view to show the copy and paste menu items that can be used for duplicating objects). See here for Raman and multiphoton transitions.

symmetric top state