If you fear that a reduced symmetry might influence the thermodynamic results, you can simply run the calculation without imposing any symmetry at all. For a 3-atom system like CO₂ the computational cost is so minimal that you can run both symmetry-free and \(D_{4h}\) calculations. Comparing the two results should give you an indication of whether the imposed symmetry has any impact on the quantities you are interested in.

Hi Jonas,

CRYSTAL does not implement point groups with infinite-order rotations, so \(D_{\infty h}\) cannot be used directly. For linear molecules like CO₂, the practical approach is to approximate the symmetry using a finite-order rotation group.

In this case, \(D_{4h}\) (i.e., 24) is a good option in CRYSTAL. It preserves the key degeneracies of linear molecules, including the doubly degenerate bending modes of CO₂. Using a lower-symmetry group like \(D_{2h}\) would artificially lift these degeneracies.

Dear Jonas,

I tried using pymatgen to extract the point-symmetry information from your .xyz file (see the Python script below):

from pymatgen.core import Molecule
from pymatgen.symmetry import analyzer

bigstructure = Molecule.from_file("yourfile.xyz")
PGstructure = analyzer.PointGroupAnalyzer(bigstructure)
sym_mol = PGstructure.get_equivalent_atoms()

print(sym_mol["eq_sets"])

This returns a Python data structure containing the symmetry-irreducible sets of atoms (only 6 for this system, which is indeed of Ih point group!).
When preparing the CRYSTAL input, be careful with the orientation of your asymmetric unit. In my case, for example, I had to change the sign of the x and y coordinates to make the symmetry consistent with CRYSTAL’s conventions.
Icosahedral point groups are available in CRYSTAL (Ih is point group number 47 in CRYSTAL), so the input fort this molecular cage reads:

Symm. structure 
MOLECULE
47
6
8       2.605032231   -11.914271806    11.762689798
6       4.344538598    15.664236912     4.366798884
6       3.427479683    14.428996321     8.326818862
6      -8.906580884     1.370529673    14.411150640             
1       2.632960331    14.962480562     7.832453804    
5      -8.776255231    -2.916256114    14.200279302     
COORPRT
TESTGEOM
END

aerba Christmas is already in the air indeed!

Dear dariag,

Yes, this might be the problem, although I was able to restart a test calculation on a small system without inserting the NODIIS keyword in the fist run. May I ask you to share your input file with us?

Dear dariag,

A static CPHF/KS calculation can be restarted from a previous run by using the RESTART keyword inside the INTCPHF block. After running some tests we have found that the RESTART option is not compatible with the use of the DIIS convergence accelerator (active by default), therefore the NODIIS keyword must be included in the input deck of the restart calculation as well, e.g.:

INTCPHF
RESTART
NODIIS
END

Every CPHF/KS run writes the necessary information for a restart to file fort.31. This file must be provided as file fort.32.

Hope this helps!

Dear Jonas,

Thanks for reaching out and being one of the most active users of these early days of the forum. Your question gives us the chance to clarify some aspects of the output file that might not be obvious to non expert users. Below, I will refer to your output file.

Harmonic Frequencies and IR intensities

To compute harmonic frequencies and IR intensities (with the default approach of the Berry phase) the input looks like:

FREQCALC
INTENS
ENDFREQ

In the output file, the following table is printed:

HHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHH
 EIGENVALUES (EIGV) OF THE MASS WEIGHTED HESSIAN MATRIX AND HARMONIC TRANSVERSE
 OPTICAL (TO) FREQUENCIES. IRREP LABELS REFER TO SYMMETRY REPRESENTATION
 ANALYSIS; A AND I INDICATE WHETHER THE MODE IS ACTIVE OR INACTIVE,
 RESPECTIVELY, FOR IR AND RAMAN; INTEGRATED IR INTENSITIES IN BRACKETS.

 CONVERSION FACTORS FOR FREQUENCIES:
      1 CM**(-1) =   0.4556335E-05 HARTREE
      1 THZ      =   0.3335641E+02 CM**(-1)

 HHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHH

    MODES         EIGV          FREQUENCIES     IRREP  IR   INTENS    RAMAN
             (HARTREE**2)   (CM**-1)     (THZ)             (KM/MOL)
    1-   1    0.3488E-07     40.9894    1.2288  (A  )   A (     0.40)   A
    2-   2    0.6077E-07     54.1037    1.6220  (A  )   A (     0.96)   A
    3-   3    0.6801E-07     57.2344    1.7158  (A  )   A (     2.45)   A
    4-   4    0.2238E-06    103.8371    3.1130  (A  )   A (    11.47)   A
    [...]

For each mode (or set of degenerate modes) its eigenvalue (in Ha\(^2\)), harmonic frequency (in cm\(^{-1}\) and THz) and irreducible representation get printed. In addition, labels specifying whether the mode is IR/Raman active are also displayed (A and I indicate whether the mode is active or inactive, respectively).

Raman intensities

Raman intensities can be computed via a coupled-perturbed approach by inserting the INTRAMAN keyword followed by the INTCPHF block in the input deck:

FREQCALC
INTRAMAN
INTCPHF
END
ENDFREQ

Raman intensities are computed for each independent component of the polarizability tensor (xx, xy, xz, yy, yz, zz, labeled as "Single Crystal" in the output file) and are also averaged to mimic polycrystalline powder samples (total, parallel polarisation, perpendicular polarisation averages are printed in the output).

POLYCRYSTALLINE ISOTROPIC INTENSITIES (ARBITRARY UNITS)

    MODES    FREQUENCIES           I_tot     I_par    I_perp
  ----------------------------------------------------------------
    1-   1       40.9894 (A  )      0.46      0.27      0.19
    2-   2       54.1037 (A  )      7.35      4.23      3.12
    3-   3       57.2344 (A  )     12.79      8.82      3.96
    4-   4      103.8371 (A  )     13.66      7.89      5.77

SINGLE CRYSTAL DIRECTIONAL INTENSITIES (ARBITRARY UNITS)

    MODES    FREQUENCIES          I_xx    I_xy    I_xz    I_yy    I_yz    I_zz
  ----------------------------------------------------------------------------
    1-   1       40.9894 (A  )    0.00    0.37    0.02    0.63    0.00    0.21
    2-   2       54.1037 (A  )    3.17    0.69    0.00    4.35    3.66   10.05
    3-   3       57.2344 (A  )    3.82    3.54    0.02    3.50    0.03   27.53
    4-   4      103.8371 (A  )    2.57    1.81    0.01   16.25    3.34   19.62

For more details on such polycrystalline averages, please refer to sections 8.4 and 8.7 of the CRYSTAL23 manual.

Raman spectrum

A continuous Raman spectrum can be simulated by use of the RAMSPEC block, as in:

FREQCALC
INTRAMAN
INTCPHF
END
RAMSPEC
END
ENDFREQ

The simulated spectrum is printed in an external file named RAMSPEC.DAT that contains several columns: column 1 with frequencies in cm\(^{-1}\), columns 2-4 with polycrystalline intensities (total, parallel, perpendicular), columns 5-10 with single crystal intensities (xx, xy, xz, yy, yz, zz).

Effect of Temperature and Laser wavelength

The effect of temperature and laser wavelength on computed Raman intensities can be accounted for by use of the RAMANEXP keyword, as in:

FREQCALC
INTRAMAN
INTCPHF
END
RAMANEXP
298 532
RAMSPEC
END
ENDFREQ

Here we set 298 K for the temperature and 532 nm for the laser wavelength. This option modifies the values of all computed Raman intensities (in the output and in the RAMSPEC.DAT file accordingly).

Please, note that other properties (harmonic frequencies and IR intensities) are not affected by this option and thus remain unchanged in the output.

Plots

When CRYSPLOT reads the CRYSTAL output file it only plots the total intensity of the polycrystalline powder model.

When CRYSPLOT reads the RAMSPEC.DAT file it plots all components:

Other plotting tools can be used to plot specific columns of the RAMSPEC.DAT file (e.g., CRYSTALClear, gnuplot).

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