Frequencies

Imaginary Frequencies

Problem: totally unexpected significant imaginary frequencies are obtained (in a frequency run) where you are pretty convinced that all frequencies should be real.

Possible cause 1: problems with the electronic configuration. If there are competing configurations, the electronic states in the different displaced geometries may be different, resulting in energies and gradients belonging to different potential energy surfaces to be compared and combined into force constants (frequencies).

Check: orbital occupations and SCF convergence behavior: if the SCFs in the displaced geometries start with large errors and/or converge very slowly you are likely to have stumbled into different configurations, so that the results from the displaced geometries are incompatible.

Cure: This is a difficult situation that may require some experimenting and judicious manipulation of the various SCF options. The bottom line is that you should try anything you can to ensure that all involved geometries have the same electronic configuration. As long as you fail to achieve this, the results are meaningless.

Possible cause 2: flat potential energy surface (think about almost free rotation modes) coupled with relatively high noise level in gradients caused by numerical integration errors or not sufficiently converged geometry optimization.

Check: visualize the imaginary frequencies in AMSspectra and check that the corresponding normal modes match motions that are expected to have a (nearly) flat energy profile.

Cure:

• restart geometry optimization with stricter convergence criteria. The default criterion on gradients of 0.001 Hartree/Angstrom may not be strict enough for some systems. In such cases a value of 0.0001 is recommended, and for accuracy reasons use “good” numerical quality and EXACTDENSITY (important for GGAs).

After the AMSification of ADF in 2020, numerical frequencies or analytical frequencies can be computed immediately after a geometry optimization. Example input with strict settings using analytical frequencies and a TZ2P basis set.

$ADFBIN/ams <<eor
   Task GeometryOptimization
   GeometryOptimization
      Convergence gradients=1e-4
   End
   Properties
      NormalModes True
   End
   NormalMode
      Hessian Analytical
   End

   Engine ADF
      NumericalQuality Good
      Basis
        Type TZ2P
        Core None
      End
      ExactDensity
   EndEngine
eor

Geometry-displacement numbers in the logfile are not contiguous

Problem: successive displaced geometries in the log file are numbered, but in your case these numbers make sudden jumps, like “0, 1, 2, 5, 6, 13…”

Cause: you’re using Cartesian displacements in a system that has some symmetry in its equilibrium geometry. The program skips the displacements of symmetry-equivalent atomic coordinates to save time. The displacement counts in the logfile do not run over the actually performed displacements but over all atomic coordinates that could be displaced if no use were made of symmetry properties.

Cure: there is no error; don’t worry.