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. The frequency dependent Raman scattering can be calculated for 1 laser frequency at the time.
RESPONSE
RAMAN
Nfreq 1
FrqBeg Laserfreq
[Optional Frequency/Energy Unit]
END
Frequencies or wavelengths
The number of frequencies Nfreq should be 1. With subkey Frqbeg the value of the Laser frequency value (Laserfreq) can be given. Default frequency unit is eV. This can be changed into Hartree units (a.u.) or in wavelengths (angstroms) by typing HARTREE or ANGSTROM on a separate line within the RESPONSE block, instead of [Optional Frequency/Energy Unit].
For static Raman scattering (ω = 0) use:
RESPONSE
RAMAN
END
The Raman scattering 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. More details on the RESPONSE key can be found here.
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 next section.
The alternative Raman implementation with the AORESPONSE offers some unique features like lifetime options.
AORESPONSE
RAMAN
END
