A basic level of a non-Boltzmann population distribution is
available through setting the vibrational temperature, Tvib,
which affects the relative populations of different vibronic
states, or the spin temperature Tspin. For more
complicated situations, such as simulating double resonance
spectra or nascent population distributions following reactions
there are two more general methods of specifying the population
of levels. It is also possible to fit these populations to
measured intensities or band contours, as the populations are
simply parameters that can be floated as any other; see the
sections on Line Intensity
Fitting and Contour Fitting.
It is also possible to simulate and fit time of flight or
kinetic energy release spectra; use IntensityUnits = PopDist to ignore
transition moments and just plot energy levels with populations
derived as described in this section. Plots against velocity (in
units of cm-1/2), rather than energy can be obtained
by Setting PlotUnits to
Where many levels are involved, a general function instead of the
simple Boltzmann equation can be used, as described under Custom Populations
Alternatively, the populations of each level can be specified
individually as described below, and is most straightforward where
only a few levels are involved.
The populations will show in the parameter area of a manifold
; by default no populations will
be set so this area will be empty. To set up population
- In the constants window, right click on the relevant
manifold or state in the treeview.
- Select "Add All Levels"; this will add all the
levels of that manifold or state (within the J range set up) and
initialize them using the current Temperature and Tvib to
calculate the population. (This can easily give an
unmanageably large number of levels, so set Jmax or
use the method below.)
- You can get a matching table of energy levels and quantum
numbers using the "List" button in the "Levels" window,
providing the same manifold or state is selected.
or, to add individual levels or sets of levels:
- Use "View","States"
to bring up the State Details Window.
- Select J, symmetry
and Manifold such that one of the states of interest is
- In the "Eigenvalues" pane (the lower matrix) select highlight
one or more levels with the mouse and then right click on the
selected levels and select "Add to population selection".
- Repeat the above steps for all the required levels.
will now show a parameter for each state at the manifold level.
The "Value" column is the relative population, excluding the
degeneracy. The parameter name is the string "J
<symmetry number> <eigenvalue number>". If external
fields are present, the parameter name will be "M
-1 <eigenvalue number>". (Both types of entries can be present,
but only one set will be active for any given simulation.)
To activate these parameters either:
If neither of these conditions are met the individual populations
will be ignored and the standard Boltzmann distribution will be
- Set UsePopParams for the required
- Set the overall Temperature <
0 to force the use of the numerical populations for all
To see details of the calculation, right click on a manifold or
state as required, and select "Level List by Symmetry" or "Level
List by Energy".
To remove the custom population entries, go to "View,Constants" and right
click on the entries in the parameter pane for the manifold. There
are items for removing selected or all items.