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.
