In ADF2006 it was already possible to request a full scan of all frequencies obtained by finite differences. This was originally done to help identify spurious imaginary frequencies that sometimes appear where one would expect a very low (nearly zero) frequency. Most frequently this happens when there is a barrier-free rotation of, for example, methyl groups.
Starting from ADF2007.01, it is possible to scan any range of frequencies calculated in the same run or found in a restart file. The input keyword used to request the scan is as follows:
SCANFREQ low high
low, high
Two values defining an interval of frequencies to scan. Frequencies that fall within the interval will be recalculated by numerical differentiation of the gradient along the respective frequency's normal mode. This means that 2*N single-point calculations with gradients will be performed, where N is the number of frequencies within the range. Imaginary frequencies are specified using negative values, which is consistent with the notation adopted within ADF.
The main advantage of this method is that single-point calculations used to obtain a force constant are performed within the same symmetry and, usually, with the same numerical integration grid, which significantly reduces the level of numerical noise and thus increases accuracy of the calculated frequency.
