Imaginary Frequencies

Problem: totally unexpected significant imaginary frequencies are obtained (in a Frequencies 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 ADFspectra and check that their respective normal modes correspond to movements that are expected to be have (nearly) flat energy profile.

Cure:

• restart geometry optimization with more strict convergence criteria. The default criterion on
  gradients 0.01 Hartree/Angstrom may be not strict enough for some systems.
  In such cases a value of 0.0001 is recommended.
• use DISHUL option of the INTEGRATION keyword to a higher value, for example 5. A higher value makes
  gradients smoothing more efficient. See also the SMOOTH subkeyword