Frequency calculations

With xtb, you can also perform Hessian calculations, which are required for thermocorrections to free energies and for computing IR and Raman spectra. To run a Hessian calculation, use the --hess flag.

xtb struc.xyz --hess

Alternatively, the --ohess flag can be used to directly perform an optimization followed by a Hessian calculation.

Warning: Computing Hessians on structures that are not fully optimized will have no physical meaning. This should generally be avoided when computing thermocontributions or spectra.

At the end of the calculation, the output will include the frequencies, total free energy, and the total thermocorrection (G(RRHO) contrib.) at the chosen temperature (default 298.15 K):

         :::::::::::::::::::::::::::::::::::::::::::::::::::::
         ::                  THERMODYNAMIC                  ::
         :::::::::::::::::::::::::::::::::::::::::::::::::::::
         :: total free energy         -97.955199823991 Eh   ::
         ::.................................................::
         :: total energy              -98.337562149475 Eh   ::
         :: zero point energy           0.443793453483 Eh   ::
         :: G(RRHO) w/o ZPVE           -0.061431127998 Eh   ::
         :: G(RRHO) contrib.            0.382362325484 Eh   ::
         :::::::::::::::::::::::::::::::::::::::::::::::::::::

To adjust the temperature, an additional input file can be provided. Even multiple temperatures are possible, e.g.:

$thermo
    temp=150.0,200.0,250.0,273.15,298.15
$end

Additionally, a vibspectrum file will be generated, containing the IR-active modes with their respective intensities. A Gaussian output file (g98.out) will also be created, which can be opened with tools like Molden to directly view the vibrational levels.

Tip: By activating the Norm. Mode in the top right of the Molden control window, a list of frequencies is shown that can be selected to display the respective vibrational mode.

These vibrational levels can be used to identify transition states or to confirm that the geometry has converged to a minimum.

Exercise

Assess the provided structure and check whether it represents a minimum, a reasonable transition state, or an arbitrary point on the potential energy surface (PES). Use GFN2-xTB with ALPB(THF). Observe how the spectrum changes when switching to GFN-FF with ALPB(THF) instead.

60

Ir 0.22444 0.14527 -0.19941
B 1.33274 -1.02240 1.19281
O 2.10728 -0.52368 2.23124
O 1.29878 -2.41761 1.23252
C 2.75593 -1.61003 2.89859
C 2.04814 -2.87405 2.36426
B 1.80738 -0.67323 -1.25750
O 1.60340 -1.37252 -2.45859
O 3.15353 -0.70050 -0.91729
C 2.83363 -1.99118 -2.85163
C 3.89810 -1.30030 -1.97774
B 1.44948 1.70801 0.27916
O 2.50688 2.19680 -0.48225
O 1.26386 2.52815 1.40609
C 3.16792 3.23950 0.23108
C 2.18823 3.61911 1.36048
C -2.30905 -0.65090 1.42723
C -2.25834 -1.68944 0.37026
N -1.30803 0.26358 1.43328
N -1.23625 -1.60021 -0.50919
C -3.32614 -0.60417 2.38997
C -3.30852 0.38423 3.36785
C -2.26730 1.30909 3.36702
C -1.28713 1.21455 2.38323
C -3.20068 -2.72376 0.27943
C -3.08457 -3.67137 -0.73143
C -2.02500 -3.56877 -1.63030
C -1.12272 -2.51872 -1.48063
C -1.14523 1.44405 -1.40055
C -1.32790 2.78803 -1.02925
C -1.99683 0.92863 -2.39248
C -2.34780 3.56470 -1.58615
C -3.01467 1.70232 -2.95766
C -3.19935 3.02499 -2.55218
H 0.37405 0.80500 -1.68399
H -0.44935 1.90106 2.33342
H -2.20575 2.09631 4.11121
H -4.09485 0.42683 4.11592
H -4.12897 -1.33084 2.37621
H -4.01648 -2.79488 0.98859
H -3.80980 -4.47600 -0.81155
H -1.89129 -4.28555 -2.43403
H -0.27298 -2.38745 -2.14226
H 1.35904 -3.30922 3.10064
H 2.74940 -3.65517 2.05006
H 2.65464 -1.48884 3.98290
H 3.82436 -1.59665 2.64805
H 1.64159 4.54652 1.14131
H 2.67739 3.72788 2.33451
H 3.38162 4.07382 -0.44625
H 4.12089 2.86119 0.62441
H 4.43772 -0.51679 -2.52565
H 4.63150 -2.00027 -1.56306
H 2.99198 -1.84055 -3.92463
H 2.77505 -3.07061 -2.65637
H -0.67931 3.23230 -0.27913
H -2.47374 4.59719 -1.26591
H -3.98804 3.63103 -2.99159
H -3.65923 1.27131 -3.72132
H -1.86324 -0.09332 -2.73789

Single-Point Hessian

Another feature of xtb is the Single-Point Hessian (SPH) approach, which allows the calculation of reasonable Hessians on arbitrary structures.

This can be useful, for example, to compute thermocorrections for a DFT-optimized structure, saving computational time compared to a more expensive DFT Hessian calculation.

To activate the SPH approach, use the --bhess flag, e.g.

xtb struc.xyz --bhess

Warning: SPH is often not suitable for transition states, as the imaginary mode will also be lost.

For further details, please refer to the documentation and the publication.