Non long‐range corrected density functionals incorrectly describe the intensity of the CH stretching band in polycyclic aromatic hydrocarbons

Abstract

We present a comprehensive study of the most relevant numerical aspects influencing frequencies and intensities in the infrared spectrum of isolated polycyclic aromatic hydrocarbons (PAHs) regarding the overestimate of the IR CH-stretching bands. We use naphthalene as benchmark and show the validity of our results to different members of the PAH family. Our analysis relies on widely employed density functional theory methods and second-order vibrational perturbational theory for the computation of vibrational eigenstates. We have focused on the elucidation of the origin of the systematic overestimate of the intensities in the CH-stretching region. To rule out nonfundamental numerical errors, we have initially considered the influence of the electronic basis set and various other parameters on the different stages of the vibrational analysis. In a second stage, we have benchmarked the results of different density functional theory functionals with respect to the aforementioned overestimate taken as the ratio between the most prominent features of the spectrum, the CH-bending and the CH-stretching bands. Our results unambiguously indicate that the long-range correction plays a major role in this spurious numerical issue. More specifically, this phenomenon is due to an incorrect description of the charge distribution (and hence dipole) within the symmetrically relevant C H bonds. Longrange correction specifically remedies this issue. It improves the description of the intensities in the stretching region while at the same time it does not perturb significantly the rest of the spectrum. With respect to the frequencies, we have observed an overall improvement when compared to noncorrected functionals.

Size distribution of polycyclic aromatic hydrocarbons in space: an old new light on the 11.2/3.3 μm intensity ratio

“…However, considering the intensities, the harmonic, relatively low-level calculations are known to consistently overestimate the CH stretch band intensity. 29 To corroborate these ndings, in Fig. 2 we compare the calculated ratio from the PAHdb with the ratio determined from gas-phase direct absorption measurements from the NIST database.…”

“…Even more so because it was noticed already in the 90s by Langhoff and later by others that the intensity ratios derived at different levels of theory can vary signicantly. [27][28][29][30] First, the use of a small basis set results in an overprediction of CH stretch intensities by about 20% when compared to matrix-isolation absorption spectra. 27 Secondly, it was recently shown that anharmonicity leading to phenomena such as combination bands and resonances has a dominant effect on the CH stretch region of the IR spectrum.…”

Automatic Determination of the Non‐Covalent Stable Conformations of the NO2‐Pyrene Cluster in Full Dimensionality (81D) Using the vdW‐TSSCDS Approach

“…Indeed, as it will be shown, the conformational changes in the reaction network are associated with weak changes in the (non‐covalent) interactions, thus implying variations of the energy of the order of tenths of wavenumbers. These slight variations, such as those involved in small‐step optimizations, are close to the numerical accuracy of the employed algorithms [26,27] . Consequently, minimizing the human‐in‐the‐loop is (almost) a necessity and automated calculations become mandatory.…”

“…[26] As for the high level, all stationary points geometries, minima, transition states, and IRC final step geometries have been optimized at the UωB97XD/cc-pVDZ level of theory, using the tight convergence criterion and a SuperFineGrid for the DFT integrals. [27,52] Notably, we use the unrestricted formalism due to the radical character of NO 2 in its ground electronic state. The proposed level of theory has been shown to be suitable for similar systems where accurate descriptions of long-range dispersion forces and exchange-correlation effects are required.…”

“…These slight variations, such as those involved in small-step optimizations, are close to the numerical accuracy of the employed algorithms. [26,27] Consequently, minimizing the human-in-the-loop is (almost) a necessity and automated calculations become mandatory. In our case, most of the calculations have been automatized according to our recent van der Waals (vdW), or non-covalent, extension [26] of the Transition State Search by Chemical Dynamics Simulation method (TSSCDS).…”