SCM: ADF User's Guide - Raman Intensities for Selected Frequencies

Raman Intensities for Selected Frequencies

The RamanRange keyword can be used to calculate Raman intensities for a range of frequencies only. Recommended to be used in the case one use a t21 as a restart file, which has frequencies on them. Using this option is a fast alternative for the existing method of calculating Raman intensities. Note that one should not use the key RAMANRANGE in combination with the key SYMMETRY. The input keyword is as follows:

RAMANRANGE low high

low, high

Two values defining an interval of frequencies to calculate the Raman intensities for. The Raman intensities are calculated by numerical differentiation of the polatizability tensor. Only frequencies within the interval that are known to be Raman-active will be included. This means that 2*N single-point TDDFT calculations will be performed, where N is the number of Raman-active frequencies within the range. Imaginary frequencies are specified using negative values, which is consistent with the notation adopted within ADF.

Main advantages of the method:

Full symmetry at each displaced geometry is used, which does not only speeds the calculation up but also makes it more accurate.
Only Raman-active modes are included in the calculation, which may save a lot of time for molecules with symmetry.
There is no need to recalculate frequencies if you already have a t21 file with them as it can be used as a restart file.

For static Raman scattering (ω = 0) one does not need to add the RESPONSE block key. However, for the calculation of the frequency dependent Raman scattering the following RESPONSE block key is needed in the input:

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].