Raman scattering intensities and depolarization ratios for all molecular vibrations at a certain laser frequency can be calculated in a single run. The run type must be Frequencies, which is arranged with the FREQUENCIES subkey of the key GEOMETRY (numerical frequencies), or with the block key ANALYICALFREQ (analytical frequencies), see IR Frequencies.
The RESPONSE key is used to specify that Raman intensities are computed.
RESPONSE
RAMAN
END
In this example the static Raman scattering is calculated (ω = 0). This type of calculation is very similar to an IR intensity calculation. In fact, all IR output is automatically generated as well. At all distorted geometries the dipole polarizability tensor is calculated. This is very time-consuming and is only feasible for small molecules.
There are a few caveats:
- Numerical integration accuracy must be high
- A calculation in which only a subset of the atoms is
displaced is not possible for Raman calculations.
- For good results, a well converged (with the same basis and functional)
equilibrium geometry must be used.
Because of this last point, it is wise to always start the RAMAN calculation with a TAPE13 restart file from a previous geometry optimization with the same basis, accuracy parameters, and density functional.
Atomic coordinate displacements in a RAMAN calculation must be Cartesian, not Z-matrix. Furthermore, the current implementation does not yet support constrained displacements, i.e. you must use all atomic coordinate displacements. However, one can calculate Raman for selected frequencies, see nexet section.
