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# Spectroscopic Constants for Diatomics

Code author: Andrew C. Simmonett

`anharmonicity`(rvals, energies)

  Generates spectroscopic constants for a diatomic molecules. Fits a diatomic potential energy curve using either a 5 or 9 point Legendre fit, locates the minimum energy point, and then applies second order vibrational perturbation theory to obtain spectroscopic constants. The r values provided must bracket the minimum energy point, or an error will result.

  A dictionary with the following keys, which correspond to spectroscopic constants, is returned:

  Parameters:

  - rvals (list) – The bond lengths (in Angstrom) for which energies are provided of length either 5 or 9 but must be the same length as the energies array
  - energies (list) – The energies (Eh) computed at the bond lengths in the rvals list

  Returns: | (dict) Keys: “re”, “r0”, “we”, “wexe”, “nu”, “ZPVE(harmonic)”, “ZPVE(anharmonic)”, “Be”, “B0”, “ae”, “De” corresponding to the spectroscopic constants in cm-1

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