Molecule Types Vibrational Structure Vibrational Energy Levels and Franck-Condon Factors

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Making a basic data file for simulating vibrational structure

A simple vibrational structure data file can be generated from File, New, Vibrational Structure. This sets up the program to simulate an electronic transition with an origin as 50000 cm^-1 and one active vibration with a ground state frequency, ω", = 1000 cm^-1 reducing to 750 cm^-1 in the excited state and displaced by 0.5 along the normal co-ordinate. To adjust the constants, add more modes or change the type of vibronic transition use 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. To set the number of vibrational modes and symmetry of the molecule click on "VibratingMolecule" and change "nModes" and "PointGroup" as required. You need only enter the number of modes that you are interested in. Note that you should leave "nNuclei" at 0 unless you specifically want to calculate Franck-Condon factors from the l matrix - see Making a data file for simulating vibrational structure from the l matrix
   
2. The symmetry of the ground electronic state is set by clicking on "X" and altering "Symmetry" as required. (The default is totally symmetric.)
3. To enter the ground state vibrations click on "v1", "v2" ... in turn under "X" - if the number that appear is wrong set "nModes" as in step 1. The key things to set are the vibrational frequency, "Omega", the symmetry of the vibrational mode, "Symmetry" and "vMax", the maximum value of the vibrational quantum number to consider in each state. Note:
   
• Setting the symmetry of a mode in one state will alter the same mode in all other electronic states of the molecule, so you do not need to change the symmetries of the vibrations in the excited state.
• To avoid a very large calculation, only set vMax non-zero for a few modes; if the product over all modes of (vMax+1) is more than 50 or so the calculation is likely to become rather slow.
  
4. The symmetry and origin of the excited electronic state is set by clicking on "A" and altering "Symmetry" and "Origin".
5. The excited state vibrational frequencies are then entered by clicking on "v1", "v2" ... in turn under "A", and setting "Omega" as required.
6. Finally the displacements along each normal mode (K) between the two electronic states are entered by clicking on the individual modes under <A|FCF|X> and setting "Displacement"