IR SubmoduleΒΆ

Note

This feature will be present in the upcoming release version 6.5.2

The IR submodule allows to evaluate IR intensities from data produced in previous calculations. To run the IR submodule use

xtb ir [options] <geometry> --dftbplus [hessian] --born [borncharges]

The geometry can be any format accepted by xtb, while the hessian format depends on the options passed to the IR submodule. Right now, only the projected DFTB+ hessian data group is available.

Mandatory keywords are:

--born

Read a DFTB+ dipole derivatives / born charges born.out file

--dftbplus

Read a DFTB+ hessian.out file, projection will be performed automatically

Please find additional keywords in the documentation for the thermo submodule. IR intensities, sometimes also called Born charges, can equivalently be obtained from

  1. mixed 2nd derivatives of the energy with respect to atomic positions and external electric field

  2. 1st derivative of dipole moment with respect to atomic positions

  3. 1st derivative of forces with respect to external electric field

In DFTB+, the second pathway is employed. This module aims to easen the calculation of SQM solid state IR spectra for large molecular crystals or organic crystals with very large unit cells, where automated supercell generation and phonon vibration evaluation is no longer computationally feasible, not even with SQM methods. Please keep in mind, that these missing lattice vibrations introduce an unkown error into the low-wavenumber area.