Vibrational Spectroscopies: IR, Raman, INS

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:

Screenshot 2025-03-13 at 12.47.37.png

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

Thank you very much Jacques and Alessandro.
Now it works :).
Best regards
Xavier

Hi,

The presence of imaginary frequencies is a sign that the geometry is not a minimum of the potential energy surface (PES). In general, this may due to two main factors:

1) A somewhat loose overall numerical precision in the geometry optimization + harmonic frequencies calculation. Here, it seems that you have already explored a few parameters. We can distinguish between parameters governing the overall numerical precision of the SCF + forces calculations, and those that are specific to the evaluation of the Hessian:

1.1) Precision of SCF+forces

One may increase a bit the thresholds for the screening of two-electron integrals (TOLINTEG keyword), switch-off the bipolar approximation (NOBIPOLA keyword), increase the shrinking factor (SHRINK keyword), use a denser grid for numerical integration of the exchange-correlation term (see XLGRID, XXLGRID keywords), tighten the convergence criteria for the SCF (setting TOLDEE to 10 or 11 for instance), tighten the convergence criteria for the geometry optimization step (see TOLDEG and TOLDEX keywords).

1.2) Numerical evaluation of the Hessian

In many cases, a more numerically stable evaluation of the Hessian is achieved by use of a two-sided finite difference approach (rather than the default one-sided approach). This can be activated with the NUMDERIV keyword within the FREQCALC input block as follows:

FREQCALC NUMDERIV 2 ENDFREQ

2) The presence of symmetry-constraints that prevent the optimizer to get to the minimum of the PES. If this is the case, removing symmetry constraints may be key to reach the minimum. This can be done by use of the SYMMREMO keyword (to be inserted in the geometry input block).

Hi,

By default, thermodynamic properties on top of harmonic frequencies are computed at room temperature. Different values of temperature can be explored by use of the TEMPERAT keyword. To restart the harmonic frequency calculation, use the RESTART keyword. An example is given below:

FREQCALC RESTART TEMPERAT 5 200 600 END

With the input above, thermodynamic properties will be computed and printed at 5 temperatures, equally spaced in the range 200-600 K.