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
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.
For a pure rotational transition only do steps 1 and 2 as above and
then:
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 button and then the
button to see a spectrum.
Molecule Types Asymmetric Tops 
button and then the 
button to see a spectrum.