A simple asymmetric top molecule file can be generated from File, New, Asymmetric Top
This initializes the program for a molecule of C2v 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:
All the settings and values listed above under 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.
Molecule Types Asymmetric Tops