Starting from ADF2007.01 it has become possible to calculate VCD spectra. The following keyword enables calculation of rotational strength during an analytical frequencies calculation:
VCD
It is important to note that the VCD keyword only works in combination AnalyticalFreq and symmetry NOSYM.
AnalyticalFreq End SYMMETRY NOSYM
The VCD intensities are calculated using Stephens' equations for VCD. For the calculation of the atomic axial tensors (AATs), analytical derivatives techniques and London atomic orbitals (the so called GIAO) are employed. As a result the calculated rotational strengths are origin independent, and therefore the common origin gauge is used [216].
Calculation of the AATs requires an analytical frequencies calculation. This limits the choice of functionals that can be used for VCD calculations. See the ANALYTICALFREQ keyword for a complete list of the available functionals. The VCD calculations can be done immediately after a geometry optimization, just like analytical frequencies calculations.
The accuracy of the vibrational rotational strengths are determined by the accuracy of the harmonic force field, atomic polar tensors (APTs) and AATs. The most critical parameter being the harmonic force field. Thus, for a fair comparison with experimental data, accurate geometries and functionals that yield accurate force fields (e.g. BP86, OLYP, etc) should be used. Our tests showed that the BP86 functional in combination with TZP basis sets is always a safe choice. For a comparison of VCD spectra calculated with various functionals (e.g BP86, OLYP, BLYP, B3PW91 and B3LYP) see [216]. Regarding the geometries, we recommend the following strict settings, 10-4 for the geometry convergence of the gradients, and integration accuracy 6. The default settings should be used for the calculation of the frequencies.
The current VCD implementation does not support symmetry and therefore symmetry NOSYM should be specified in the input file. The frozen core approximation and open shell systems are also not supported.
By default, only the vibrational rotational strengths are printed in the ADF output file. The AATs can also be printed by specifying the keyword:
PRINT VCD
