Molecule Types Asymmetric Tops <Prev Next>

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 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:
  1. At the Molecule level (Click on "AsymmetricTop") the PointGroupRepresentation 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.
  4. The transition moment may also need adjusting - click on "<v=1|a|v=0>" . Key things to adjust are:
For details of the Hamiltonian used, see the section on the Asymmetric Top Hamiltonian.

Pure Rotational (Microwave) Spectra

For a pure rotational transition only do steps 1 and 2 as above and then:
  1. 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.