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 C3v
symmetry with a
band origin of 1500 cm-1
and with rotational constants
in both states of A
, B =
. To adjust the constants (and set the type of
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
- At the Molecule level
(Click on "SymmetricTop")
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 C3v.
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
- The rotational
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.
- For degenerate (E symmetry) vibronic states, the Coriolis
coupling constant, ζ(=zeta)
is also essential. This is a dimensionless constant, typically
in the range -1 .. +1.
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.
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
For details of the Hamiltonian used, see the symmetric top state section.
Pure Rotational (Microwave) Spectra
For a pure rotational transition only do steps 1 and 2 as above and
- 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.)
- The upper state can be deleted - right click on "Excited" and select
- 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.