Frequencies

Harmonic frequencies are computed in ADF by numerical differentiation of energy gradients in slightly displaced geometries [12,13]. The computation of frequencies is activated by specifying it as the runtype in the geometry block, see the Geometry Optimization section above.

An alternative is available in the post-ADF property program SD. It analytically calculates the second derivatives. Although the SD program is planned to become the recommended option in the future, it is currently still in the development stage and not yet very efficient. At the moment the FREQUENCIES keyword is therefore still the recommended option for general use. In special or difficult cases, the SD program may already be of use though. Please check the 'ADF Property Programs' documentation for further details on the SD program.

Most of the subkeys in the geometry block are meaningless for the calculation of frequencies. Indeed, a Frequencies calculation is not a variation on optimization, but rather a sequence of Single Point runs for the equilibrium geometry and a series of slightly different geometries. By comparison of the computed gradients the force constants and hence the frequencies are computed (in the harmonic approximation of the energy surface).

GEOMETRY
 Frequencies {Numdif=Numdif} {Disrad=drad} {Disang=dang}
 iterations Niter
end

Numdif

Must have the value 1 or 2 and specifies the type of numerical differentiation that is applied to compute the force constants from gradients in slightly displaced geometries: 1-point or 2-point numerical differentiation. In the former case the gradients of the displaced geometry are compared with the gradients at the input (equilibrium) geometry. In the latter case both a negative and a positive displacement are applied, yielding much more accurate results but at the expense of more computations. Default: numdif=2.

dang and drad

The displacements of the coordinates that will be varied. Dang applies to angles (bond and dihedral) in degrees and drad applies to Cartesian (x, y, z) coordinates and to bond lengths, in angstrom. Defaults: 1 degree and 0.01 angstrom.

Niter

In a calculation of frequencies it is the total number of (displaced) geometries for which gradients are computed. By default this is internally determined such that the calculation of frequencies can be completed. If you reduce it, the run will only partially build the matrix of force constants and a restart is required to complete the computation.

WARNING: you cannot combine a Frequencies calculation with the QMMM feature.