First Steps with SPFIT/SPCAT
SPFIT and SPCAT are two programs developed by Herbert Pickett. SPFIT fits the effective Hamiltonian to assignments that connect the experimental lines with the correct quantum numbers. SPCAT then creates predictions from the effective Hamiltonian. There are several programs to generate the assignments of the experimental transition frequencies to the quantum numbers. In this course, we will use the program LLWP.
SPFIT takes a *.lin containing the assignments and a *.par file that defines the initial values for the effective Hamiltonian as input. It creates a backup file with the extension *.bak of the *.par file and updates the *.par file with the new best parameters. A summary of the fit is reported in the *.fit file and the parameters and their uncertainties are written to the *.var file. SPCAT then uses the *.var file and the *.int file (holding the dipole moments and other intensity-related parameters) to create the predictions in *.cat format. A summary of the predictions is provided in the *.out file. If requested the energy levels are reported in the *.egy file and the transition dipole moments in the *.str file.
For the formats of the different file types, see the official documentation and its annotated version on the PROPSE page.
To summarize the different file types:
| File | Content |
|---|---|
| *.lin | Assignments of the experimental center positions to their respective quantum numbers |
| *.par | The parameters of the effective Hamiltonian (rotational constants, centrifugal distortion constants, …) and any settings connected to the symmetry of the molecule |
| *.fit | Summary of the SPFIT run |
| *.bak | Backup of the *.par file before running SPFIT; Revert to this file if your model does not converge or explodes |
| *.var | Similar to the *.par file but includes the uncertainty of each parameter |
| *.int | Dipole moments, partition function, temperature, … |
| *.cat | Predicted transitions |
| *.egy | Predicted energy levels |
| *.str | Predicted transition dipole moments |
| *.out | Summary of the SPCAT run |
Secondary literature on these programs exists (e.g. by Brian Drouin) and worked examples of similar molecules are often the most helpful resources.
How to Start
For most use cases it makes sense to first set up an initial model. Starting values can either be derived from simple assumptions (as seen in the previous section) or from quantum chemical calculations. Running SPCAT on this initial model yields predictions that match the experimental spectrum qualitatively, but not quantitatively. Then spectroscopists like us have to correctly assign the predictions to the lines in the spectrum. Once the assignments have been made, we can refine the model by running SPFIT and subsequently SPCAT, which will create new predictions. This begins a process of iterative refinement, whereby more transitions are assigned and the model is further refined.
