VIBFREQ1295: A New Database for Vibrational Frequency Calculations

Abstract

High-throughput approaches for producing approximate vibrational spectral data for molecules of astrochemistry interest rely on harmonic frequency calculations using computational quantum chemistry. However, model chemistry recommendations (i.e., a level of theory and basis set pair) for these calculations are not yet available and, thus, thorough benchmarking against comprehensive benchmark databases is needed. Here, we present a new database for vibrational frequency calculations (VIBFREQ1295) storing 1295 experimental fundamental frequencies and CCSD­(T)­(F12*)/cc-pVDZ-F12 ab initio harmonic frequencies from 141 molecules. VIBFREQ1295’s experimental data was complied through a comprehensive review of contemporary experimental data, while the ab initio data was computed here. The chemical space spanned by the molecules chosen is considered in-depth and is shown to have good representation of common organic functional groups and vibrational modes. Scaling factors are routinely used to approximate the effect of anharmonicity and convert computed harmonic frequencies to predicted fundamental frequencies. With our experimental and high-level ab initio data, we find that a single global uniform scaling factor of 0.9617(3) results in median differences of 15.9(5) cm–1. A far superior performance with a median difference of 7.5(5) cm–1 can be obtained, however, by using separate scaling factors (SFs) for three regions: frequencies less than 1000 cm–1 (SF = 0.987(1)), between 1000 and 2000 cm–1 (SF = 0.9727(6)), and above 2000 cm–1 (SF = 0.9564(4)). This sets a lower bound for the performance that could be reliably obtained using scaling of harmonic frequency calculations to predict experimental fundamental frequencies. VIBFREQ1295’s most important purpose is to provide a robust database for benchmarking the performance of any vibrational frequency calculations. VIBFREQ1295 data could also be used to train machine-learning models for the prediction of vibrational spectra and as a reference and data starting point for more detailed spectroscopic modeling of particular molecules. The database can be found as part of the Supporting Information for this paper or in the Harvard DataVerse at .

Model Chemistry Recommendations for Scaled Harmonic Frequency Calculations: A Benchmark Study

“…Instead, we find that median errors are generally a more appropriate statistical metric to judge model chemistry performance, providing a single number description of the distribution of errors in the scaled harmonic frequencies. To further assess the distribution of errors for important model chemistries, we also use box-and-whisker plots and quantify the first, second (median error), and third quartiles.…”

“…Comparisons between the experimental and high-level ab initio data in VIBFREQ1295 allowed us to define and further understand the best approach to assess model chemistry performance through the use of harmonic frequency scaling factors. We found that a frequency-range-specific scaling approach, specifically dividing the frequency range into three different frequency regions (low (1,000 cm –1 ), mid (1,000–2,000 cm –1 ), and high (≥2,000 cm –1 )), each one with a corresponding scaling factor, leads to significantly improved performance compared to that of the traditional global scaling approach (corresponding to a single value to scale all frequencies in the spectrum) or a high- and low-frequency division. We also found no benefit in optimizing scaling factors for different molecular groups (e.g., halogen- and non-halogen-containing molecules) or vibrational mode types (e.g., stretches and nonstretches) .…”

“…We found that a frequency-range-specific scaling approach, specifically dividing the frequency range into three different frequency regions (low (1,000 cm –1 ), mid (1,000–2,000 cm –1 ), and high (≥2,000 cm –1 )), each one with a corresponding scaling factor, leads to significantly improved performance compared to that of the traditional global scaling approach (corresponding to a single value to scale all frequencies in the spectrum) or a high- and low-frequency division. We also found no benefit in optimizing scaling factors for different molecular groups (e.g., halogen- and non-halogen-containing molecules) or vibrational mode types (e.g., stretches and nonstretches) . Moreover, our results showed that median errors are more appropriate than RMSEs in describing the distribution of errors and predicting performance of model chemistries in new calculations given that RMSEs are dominated by outliers rather than typical expected results.…”

“…Benchmark databases play a crucial role in optimizing scaling factors and, consequently, assessing model chemistry performance. With the most extensive and frequently used database from Pople et al (also known as the F1 set as used in Scott and Radom) collated almost 30 years ago, we recently compiled a new and updated benchmark database for vibrational frequency calculations (VIBFREQ1295) storing 1,295 experimental fundamental frequencies and CCSD­(T)­(F12*)/cc-pVDZ-F12 , ab initio harmonic frequencies for 141 organic-like molecules . The development of VIBFREQ1295 was based on an extensive compilation of contemporary experimental data (usually at high resolution), providing general updates to previous molecular frequency assignments.…”

10 Years of the ACS PHYS Astrochemistry Subdivision

“…43 One manuscript in this virtual special issue shows that resistively heated silicon carbide microreactors have little-to-no rovibrational cooling in the expansions, but the chemical composition of the background likely will differ from that of the molecular beam emanating directly from the reactor. 44 Finally, two spectral databases are also reported in order to provide large amounts of spectral data for many molecules 45 and rich spectral characterization of CO 2 . 46 All of these considerations for spectral analysis certainly enhance inferences that can be gleaned from these types of experiments and theoretical computations informing future astrochemical observations.…”