# Frequency

`frequency`(name[, func, mode, dertype, irrep, molecule])

  Function to compute harmonic vibrational frequencies.

  Aliases: | frequencies(), freq()
  Returns: | (float) Total electronic energy in Hartrees.

  Note

  Analytic hessians are not available. Frequencies will proceed through finite differences according to availability of gradients or energies.

  Caution

  Some features are not yet implemented. Buy a developer a coffee.

  - Implement sow/reap mode for finite difference of gradients. Presently only for findif of energies.

  Parameters:

  - name (string) –

    `'scf'` || `'df-mp2'` || `'ci5'` || etc.

    First argument, usually unlabeled. Indicates the computational method to be applied to the system.

  - dertype (dertype) –

    `'hessian'` || `'gradient'` || `'energy'`

    Indicates whether analytic (if available- they’re not), finite difference of gradients (if available) or finite difference of energies is to be performed.

  - mode (string) –

    `'continuous'` || `'sow'` || `'reap'`

    For a finite difference of energies or gradients frequency, indicates whether the calculations required to complet the frequency are to be run in one file (`'continuous'`) or are to be farmed out in an embarrassingly parallel fashion (`'sow'`/`'reap'`)/ For the latter, run an initial job with `'sow'` and follow instructions in its output file.

  - irrep (int or string) –

    `-1` || `1` || `'b2'` || `'App'` || etc.

    Indicates which symmetry block (Cotton ordering) of vibrational frequencies to be computed. `1`, `'1'`, or `'a1'` represents , requesting only the totally symmetric modes. `-1` indicates a full frequency calculation.

  - molecule (molecule) –

    `h2o` || etc.

    The target molecule, if not the last molecule defined.

  Examples: |

      >>> # [1] <example description>
      >>> <example python command>

      >>> # [2] Frequency calculation for b2 modes through finite difference of gradients
      >>> frequencies('scf', dertype=1, irrep=4)