Making a Data File for an Asymmetric Top Molecule

A simple asymmetric top molecule file can be generated from File, New, Asymmetric Top This initializes the program for a molecule of C_2v symmetry with a band origin of 2000 cm^-1and with rotational constants in both states of A = 9 cm^-1, B = 1.2 cm^-1 and C = 1 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 "AsymmetricTop") the PointGroup, Representation and the position of the symmetry elements within the molecule (controlled by C2zAxis and C2xAxis) are critical - see Axis systems and symmetries for asymmetric tops. If you have equivalent nuclei the statistical weights (eeWt, eoWt, oeWt and ooWt) may also need setting. 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
The overall electronic spin, S
The rotational constants, A, B and C.

Now switch to the upper state (click on the "v=1" entry under "Excited") . The key things to set here are:

The Origin
All the settings and values listed above under the ground state.

The transition moment may also need adjusting - click on "<v=1|a|v=0>" . Key things to adjust are:

The axis of the dipole moment - the default is along the a axis, and can be changed with the Axis setting. The axis is likely to change if you change the symmetry or point ground of the molecule.
For low symmetry asymmetric tops more than one transition moment may be needed; either:

right click on the transition moments object (<Excited|mu|Ground>) in the tree view and select "Add New...","Cartesian Transition Moment" (or other type)
or right click on the transition moment (<v=1|a|v=0>) and select copy and then paste to produce a duplicate transition moment that can be adjusted.

Raman and multiphoton transitions are covered in a separate section.

Asymmetric Top Hamiltonian.

Pure Rotational (Microwave) Spectra

Right click on <Excited|mu|Ground> (the transition moments object) and select rename. Change the "Bra" entry in the rename dialog so that both entries read "Ground". (This converts the transition moment to act within the ground state, rather than connecting two different states.) Adjustment to the transition moment may also be required - see step 4 above.

The upper state can be deleted - right click on "Excited" and select delete.

The default frequency range for the plot will still be in the infra-red; press the simulate

button and then the All

button to see a spectrum.