Numerical frequencies (Hessian)

By specifying the runtype Frequencies you can calculate the Hessian which in turn yields the harmonic frequencies and vibrational modes. The Hessian is an important product that can be used in a geometry optimization, and in particular to start a transition state search. The second derivative of the energy (i.e. the Hessian) is obtained by numerically differentiating the analytical gradients. When your unit cell has N atoms in the unit cell and no symmetry (other than translational symmetry) the number of displacements is 2x3xN. For large systems this can be very time consuming, and you could consider to calculate only a partial Hessian. If you have a very symmetric unit cell, but are interested in asymmetric modes, set the subkeys useA1Displacements and doA1Projection to false. NB: the lattice vectors are assumed to be fixed.

A frequency run is invoked with

RunType
Frequencies
End

The rest is controlled by the Frequencies key.

Frequencies (block-type)

Key to control the numerical frequency run

Frequencies
  Step size
  useA1Displacements {true | false}
  doA1Projection {true | false}
End

Step: The step size (angstrom) for the numerical differentiation

useA1Displacements: Determine only the total symmetric modes (default = true). When disabled, NOSYM calculations will be performed.

doA1Projection: Symmetrize the Hessian (default = true). For most users the only sensible setting is to make it equal to the value of useA1Displacements.

This options honors the SelectedAtoms key, in which case only the hessian will be calculated for the selected atoms.