arXiv subjects

Laura K. McKemmish

Publications and source records attributed to Laura K. McKemmish.

At least 19 recordsLinked to original sources

arXiv2024

Quartz Clouds in the Dayside Atmosphere of the Quintessential Hot Jupiter HD 189733 b

Recent mid-infrared observations with JWST/MIRI have resulted in the first direct detections of absorption features from silicate clouds in the transmission spectra of two transiting exoplanets, WASP-17 b and WASP-107 b. In this paper, we measure the mid-infrared ($5-12$ $μ$m) dayside emission spectrum of the benchmark hot Jupiter HD 189733 b with MIRI LRS by combining data from two secondary eclipse observations. We confirm the previous detection of H$_2$O absorption at 6.5 $μ$m from Spitzer/IRS and additionally detect H$_2$S as well as an absorption feature at 8.7 $μ$m in both secondary eclipse observations. The excess absorption at 8.7 $μ$m can be explained by the presence of small ($\sim$0.01 $μ$m) grains of SiO$_2$[s] in the uppermost layers of HD 189733 b's dayside atmosphere. This is the first direct detection of silicate clouds in HD 189733 b's atmosphere, and the first detection of a distinct absorption feature from silicate clouds on the day side of any hot Jupiter. We find that models including SiO$_2$[s] are preferred by $6-7σ$ over clear models and those with other potential cloud species. The high altitude location of these silicate particles is best explained by formation in the hottest regions of HD 189733 b's dayside atmosphere near the substellar point. We additionally find that HD 189733 b's emission spectrum longward of 9 $μ$m displays residual features not well captured by our current atmospheric models. When combined with other JWST observations of HD 189733 b's transmission and emission spectrum at shorter wavelengths, these observations will provide us with the most detailed picture to date of the atmospheric composition and cloud properties of this benchmark hot Jupiter.

astro-ph.EP↗

arXiv2024

A hybrid approach to generating diatomic line lists for high resolution studies of exoplanets and other hot astronomical objects: Updates to ExoMol MgO, TiO and VO line lists

The best molecular line lists for astrophysical applications today require both high accuracy of line positions for strong lines as well as high overall completeness. The former is required to enable, for example, molecular detection in high-resolution cross-correlation observations of exoplanets, while completeness is required for accurate spectroscopic and radiative properties over broad temperature and spectral ranges. The use of empirical energies generated with the Marvel procedure is a standard way to improve accuracy; here we explore methods of extending the use of these levels using predicted shifts and isotopologue extrapolation, as well augmenting the levels from other sources such as effective Hamiltonian studies. These methods are used to update ExoMol line lists for the main 24Mg16O and 48Ti16O isotopologues, as well as for 24Mg17O, 24Mg18O, 25Mg16O, 26Mg16O, 46Ti16O, 47Ti16O, 49Ti16O and 50Ti16O; new Marvel results for 51V16O are also presented

astro-ph.SR↗

arXiv2024

Benchmarking Basis Sets for Density Functional Theory Thermochemistry Calculations: Why unpolarised basis sets and the polarised 6-311G family should be avoided

Basis sets are a crucial but often largely overlooked choice when setting up quantum chemistry calculations. The choice of basis set can be critical in determining the accuracy and calculation time of your quantum chemistry calculations. Clear recommendations based on thorough benchmarking are essential, but not readily available currently. This study investigates the relative quality of basis sets for general properties by benchmarking basis set performance for a diverse set of 136 reactions (from the diet-150-GMTKN55 dataset). In our analysis, we find the distributions of errors are often significantly non-Gaussian, meaning that the joint consideration of median errors, mean absolute errors and outlier statistics is helpful to provide a holistic understanding of basis set performance. Our direct comparison of performance between most modern basis sets provides quantitative evidence for basis set recommendations that broadly align with the established understanding of basis set experts and is evident in the design of modern basis sets. For example, while zeta is a good measure of quality, it is not the only determining factor for an accurate calculation with unpolarised double and triple-zeta basis sets (like 6-31G and 6-311G) having very poor performance. Appropriate use of polarisation functions (e.g. 6-31G*) is essential to obtain the accuracy offered by double or triple zeta basis sets. In our study, the best performance in our study for double and triple zeta basis set are 6-31++G** and pcseg-2 respectively. The polarised 6-311G basis set family has poor parameterisation which means its performance is more like a double-zeta than triple-zeta basis set. All versions of the 6-311G basis set family should be avoided entirely for valence chemistry calculations moving forward.

physics.chem-ph↗

arXiv2024

On the Specialisation of Gaussian Basis Sets for Core-Dependent Properties

Despite the fact that most quantum chemistry basis sets are designed for accurately modelling valence chemistry, these general-purpose basis sets continue to be widely used to model core-dependent properties. Core-specialised basis sets are designed with specific features to accurately represent the behaviour of the core region. This design typically incorporates Gaussian primitives with higher exponents to capture core behaviour effectively, as well as some decontraction of basis functions to provide flexibility in describing the core electronic wave function. The highest Gaussian exponent and the degree of contraction for both $s$- and $p$-basis functions effectively characterise these design aspects. In this study, we compare the design and performance of general-purpose basis sets against several literature basis sets specifically designed for three core-dependent properties: J coupling constants, hyperfine coupling constants, and magnetic shielding constants (used for calculating chemical shifts). Our findings consistently demonstrate a significant reduction in error when employing core-specialised basis sets, often at a marginal increase in computational cost compared to the popular 6-31G** basis set. Notably, for expedient calculations of J coupling, hyperfine coupling and magnetic shielding constants, we recommend the use of the pcJ-1, EPR-II, and pcSseg-1, basis sets respectively. For higher accuracy, the pcJ-2, EPR-III, and pcSseg-2 basis sets are recommended.

physics.chem-ph↗

arXiv2024

The 2024 release of the ExoMol database: molecular line lists for exoplanet and other hot atmospheres

The ExoMol database (www.exomol.com) provides molecular data for spectroscopic studies of hot atmospheres. These data are widely used to model atmospheres of exoplanets, cool stars and other astronomical objects, as well as a variety of terrestrial applications. The 2024 data release reports the current status of the database which contains recommended line lists for 91 molecules and 224 isotopologues giving a total of almost 10$^{12}$ individual transitions. New features of the database include extensive "MARVELization" of line lists to allow them to be used for high resolutions studies, extension of several line lists to ultraviolet wavelengths, provision of photodissociation cross sections and extended provision of broadening parameters. Some of the in-house data specifications have been rewritten in JSON and moved to conformity with other international standards. Data products, including specific heats, a database of lifetimes for plasma studies, and the ExoMolHR web app which allows exclusively high resolution data to be extracted, are discussed.

astro-ph.GA↗

arXiv2024

Full spectroscopic model and trihybrid experimental-perturbative-variational line list for NH

Imidogen (NH) is a reactive molecule whose presence in astrochemical environments is of interest due to its role in the formation of nitrogen-containing molecules and as a potential probe of nitrogen abundance. Spectroscopic NH monitoring is useful for Earth-based combustion and photolysis processes of ammonia and other nitrogen-containing species. NH is also relevant to ultracold molecular physics and plasma studies. To enable these diverse applications, high-quality molecular spectroscopic data is required. Here, a new line list with significant advantages over existing data is presented. Most notably, this line list models isotopologue spectroscopy and forbidden transitions (important for NH visible absorption), alongside some overall improvements to accuracy and completeness. This approach takes advantage of existing experimental data (from a previous MARVEL compilation) and perturbative line lists together with new MRCI ab initio electronic data. These are used to produce a novel variational spectroscopic model and trihybrid line list for the main 14N1H isotopologue, as well as isotopologue-extrapolated hybrid line lists for the 14N2H, 15N1H and 15N2H isotopologues. The new 14N1H ExoMol-style trihybrid line list, kNigHt, comprises 4,076 energy levels (1,078 experimental) and 327,014 transitions up to 47,500 cm-1 (211 nm) between five low-lying electronic states (X 3Σ-, a 1Δ, b 1Σ+, A 3Π and c 1Π). For most anticipated applications aside from far-IR studies, this line list will be of sufficient quality; any improvements should focus on the b 1Σ+ energies, and the a 1Δ - A 3Π and b 1Σ+ - A 3Π spin-orbit couplings.

astro-ph.EP↗

arXiv2023

Full Spectroscopic Model and Trihybrid Experimental-Perturbative-Variational Line List for ZrO

Zirconium monoxide (ZrO) absorption lines define rare S-type stars and are currently being sought on exoplanets. Successful detection is dependent on an accurate and comprehensive line list, with existing data not ideal for many applications. Specifically, the Plez \etal{} line list is near-complete but has insufficient accuracy for high-resolution cross-correlation, while the Sorensen \& Bernath data has high accuracy but only considers a small number of spectral bands. This article presents a novel spectroscopic model, variational line list and trihybrid line list for the main \ZrO{} isotopologue, as well as isotopologue-extrapolated hybrid line lists for the \isoa{}, \isob{}, \isoc{}, \isod{}~and \isoe{} isotopologues. These were constructed using \DUO{} based on icMRCI-SD/CASSCF~\abinitio{} electronic data calculated using \MOLPRO{}, experimental energies obtained from a previous \Marvel{} data compilation and perturbative energies from Sorensen \& Bernath. The new \ZrO{} \EXOMOL{}-style trihybrid line list, \LLname{}, comprises \noenergies{} energies (\noMaenergies{} experimental) and \notransitions{} transitions up to 30,000~\cm{} (333~nm) between ten low-lying electronic states (\ZrOX{}, \ZrOaa{}, \ZrOA{}, \ZrObb{}, \ZrOB{}, \ZrOC{}, \ZrOdd{}, \ZrOee{}, \ZrOff{} and \ZrOF{}). The inclusion of experimental energy levels in \LLname{} means ZrO will be much easier to detect using high-resolution ground-based telescopes in the 12,500 -- 17,500~\cm{} (571 -- 800~nm) spectral region. The inclusion of variational energy levels means that the ZorrO line list has very high completeness and can accurately model molecular absorption cross-sections even at high temperatures. The \LLname{} data will hopefully facilitate the first detection of ZrO in the atmosphere of a hot Jupiter exoplanet, or alternatively more conclusively exclude its presence.

astro-ph.EP↗

arXiv2023

High-throughput Quantum Chemistry: Empowering the Search for Molecular Candidates behind Unknown Spectral Signatures in Exoplanetary Atmospheres

The identification of molecules in exoplanetary atmospheres is only possible thanks to the availability of high-resolution molecular spectroscopic data. However, due to its intensive and time-consuming generation process, at present, only on order 100 molecules have high-resolution spectroscopic data available, limiting new molecular detections. Using routine quantum chemistry calculations (i.e., scaled harmonic frequency calculations using the B97-1/def2-TZVPD model chemistry with median errors of 10cm-1), here we present a complementary high-throughput approach to rapidly generate approximate vibrational spectral data for 2743 molecules made from the biologically most important elements C, H, N, O, P and S. Though these data are not accurate enough to enable definitive molecular detections and does not seek to replace the need for high-resolution data, it has powerful applications in identifying potential molecular candidates responsible for unknown spectral features. We explore this application for the 4.1 micron (2439cm-1) feature in the atmospheric spectrum of WASP-39b, listing potential alternative molecular species responsible for this spectral line, together with SO2. Further applications of this big data compilation also include identifying molecules with strong absorption features that are likely detectable at quite low abundances, and training set for machine learning predictions of vibrational frequencies. Characterising exoplanetary atmospheres through molecular spectroscopy is essential to understand the planet's physico-chemical processes and likelihood of hosting life. Our rapidly generated quantum chemistry big data set will play a crucial role in supporting this understanding by giving directions into possible initial identifications of the more unusual molecules to emerge.

astro-ph.EP↗

arXiv2023

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

Despite the popularity of scaled harmonic frequency calculations in chemistry, sparse benchmarking is available to guide users on appropriate level of theory and basis set choices (model chemistry). Here, we assess the performance of over 600 model chemistries in scaled harmonic frequencies calculations evaluating different scaling factors types. We can summarise our results into three main findings: (1) using model chemistry-specific scaling factors optimised for three different frequency regions (low (below 1,000cm-1), mid (1,000-2,000cm-1), and high (above 2,000cm-1)) results in better agreement between the scaled harmonic and experimental frequencies compared to other choices; (2) larger basis sets and more robust levels of theory generally lead to superior performance; however, the particular model chemistry choice matters; and (3) outliers should be expected in routine calculations. In terms of specific recommendations, overall, the highest accuracy model chemistries are double-hybrid functionals with non-Pople augmented triple-zeta basis sets, which can produce median frequency errors of down to 7.6cm-1 (DSD-PBEP86/def2-TZVPD) lying close to the error in the harmonic approximation. Double-zeta basis sets should not be used with double-hybrid functionals as there is no improvement compared to hybrid functionals. Note that the 6-311G* and 6-311+G* bases perform like double-zeta basis sets for vibrational frequencies. After scaling, all studied hybrid functionals with non-Pople triple-zeta basis sets will produce median errors of less than 15cm-1, with the best result of 9.9cm-1 with B97-1/def2-TZVPD. Appropriate matching of double-zeta basis sets with hybrid functionals can produce high quality results. The B97-1, TPSS0-D3(BJ) or wB97X-D hybrid functionals with 6-31G*, pc-1 or pcseg-1 are recommended for fast routine calculations, all delivering median errors of 11-12cm-1.

physics.chem-ph↗

arXiv2023

VIBFREQ1295: A New Database for Vibrational Frequency Calculations

High-throughput approaches for producing approximate vibrational spectral data for molecules of astrochemistry interest rely on scaled harmonic frequency calculations. However, level of theory and basis set pair recommendations for these calculations are not yet available and thus benchmarking against comprehensive benchmark databases is needed. Here, we present a new database for vibrational frequency calculations (VIBFREQ1295) storing 1,295 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 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 global scaling factor of 0.9617 results in median errors of 15.9 cm-1. Far superior performance with a median error of 7.5 cm-1 can be obtained, however, by using separate scaling factors for three regions: frequencies less than 1000 cm-1 (0.987), between 1000 and 2000 cm-1 (0.9727) and above 2000 cm-1 (0.9564). This sets a lower bound for the performance of level of theory and basis set pairs in scaled harmonic frequency calculations. VIBFREQ1295's most important purpose is to provide a robust benchmarking database for vibrational frequency calculations. The database can be found as part of the supplemental material for this paper, or in the Harvard DataVerse at https://doi.org/10.7910/DVN/VLVNU7.

physics.chem-ph↗

arXiv2021

Meta-analysis of Uniform Scaling Factors for Harmonic Frequency Calculations

Vibrational frequency calculations performed under the harmonic approximation are widespread across chemistry. However, it is well-known that the calculated harmonic frequencies tend to systematically overestimate experimental fundamental frequencies; a limitation commonly overcome with multiplicative scaling factors. In practice, multiplicative scaling factors are derived for each individual model chemistry choice (i.e., a level of theory and basis set pair), where performance is judged by, for example, the root-mean square error (RMSE) between the predicted scaled and experimental frequencies. However, despite the overwhelming number of scaling factors reported in the literature and model chemistry approximations available, there is little guidance for users on appropriate model chemistry choices for harmonic frequency calculations. Here, we compile and analyse the data for 1495 scaling factors calculated using 141 levels of theory and 109 basis sets. Our meta-analysis of this data shows that scaling factors and RMSE approach convergence with only hybrid functionals and double-zeta basis sets, with anharmonicity error already dominating model chemistry errors. Noting inconsistent data and the lack of independent testing, we can nevertheless conclude that a minimum error of 25cm-1 -- arising from insufficiently accurate treatment of anharmonicity -- is persistent regardless of the model chemistry choice. Based on the data we compiled and cautioning the need for a future systematic benchmarking study, we recommend wB97X-D/def2-TZVP for most applications and B2PLYP/def2-TZVPD for superior intensity predictions. With a smaller benchmark set, direct comparison strongly prefers wB97X-D/6-31G* to B3LYP/6-31G*.

physics.chem-ph↗

arXiv2021

Full Spectroscopic Model and Trihybrid Experimental-Perturbative-Variational Line List for CN

Accurate line lists are important for the description of the spectroscopic nature of small molecules. While a line list for CN (an important molecule for chemistry and astrophysics) exists, no underlying energy spectroscopic model has been published, which is required to consider the sensitivity of transitions to a variation of the proton-to-electron mass ratio. Here we have developed a Duo energy spectroscopic model as well as a novel hybrid style line list for CN and its isotopologues, combining energy levels that are derived experimentally (Marvel), using the traditional/perturbative approach (Mollist), and the variational approach (from a Duo spectroscopic model using standard ExoMol methodology). The final Trihybrid ExoMol-style line list for 12C14N consists of 28,004 energy levels (6,864 experimental, 1,574 perturbative, the rest variational) and 2,285,103 transitions up to 60,000 cm-1 between the three lowest electronic states (X2Sigma+, A2Pi and B2Sigma+). The spectroscopic model created is used to evaluate CN as a molecular probe to constrain the variation of the proton-to-electron mass ratio; no overly promising sensitive transitions for extragalactic study were identified.

physics.chem-ph↗

arXiv2021

Computational Infrared Spectroscopy of 958 Phosphorus-bearing Molecules

Phosphine is now well established as a biosignature, which has risen to prominence with its recent tentative detection on Venus. To follow up this discovery and related future exoplanet biosignature detections, it is important to spectroscopically detect the presence of phosphorus-bearing atmospheric molecules that could be involved in the chemical networks producing, destroying or reacting with phosphine. We start by enumerating phosphorus-bearing molecules (P-molecules) that could potentially be detected spectroscopically in planetary atmospheres and collecting all available spectral data. Gaseous P-molecules are rare, with speciation information scarce. Very few molecules have high accuracy spectral data from experiment or theory; instead, the best available data is from the RASCALL approach and obtained using functional group theory. Here, we present a high-throughput approach utilising established computational quantum chemistry methods (CQC) to produce a database of approximate infrared spectra for 958 P-molecules. These data are of interest for astronomy and astrochemistry (importantly identifying potential ambiguities in molecular assignments), improving RASCALL's underlying data, big data spectral analysis and future machine learning applications. However, this data will probably not be sufficiently accurate for secure experimental detections of specific molecules within complex gaseous mixtures in laboratory or astronomy settings.

astro-ph.EP↗

arXiv2021

Molecular diatomic spectroscopy data

Accurate and comprehensive diatomic molecular spectroscopic data have long been vital in a wide variety of applications for measuring and monitoring astrophysical, industrial and other gaseous environments. These data are also used extensively for benchmarking quantum chemistry and applications from quantum computers, ultracold chemistry and the search for physics beyond the standard model. Useful data can be highly detailed like line lists or summative like molecular constants, and obtained from theory, experiment or a combination. There are plentiful (though not yet sufficient) data available, but these data are often scattered. For example, molecular constants have not been compiled since 1979 despite the existing compilation still being cited more than 200 times annually. Further, the data are interconnected but updates in one type of data are not yet routinely applied to update interconnected data: in particular, new experimental and ab-initio data are not routinely unified with other data on the molecule. This paper provide information and strategies to strengthen the connection between data producers (e.g. ab-initio electronic structure theorists and experimental spectroscopists), data modellers (e.g. line list creators and others who connect data on one aspect of the molecule to the full energetic and spectroscopic description) and data users (astronomers, chemical physicists etc). All major data types are described including their source, use, compilation and interconnectivity. Explicit advice is provided for theoretical and experimental data producers, data modellers and data users to facilitate optimal use of new data with appropriate attribution.

physics.chem-ph↗

arXiv2020

An update to the MARVEL dataset and ExoMol line list for 12C2

The spectrum of dicarbon (C2) is important in astrophysics and for spectroscopic studies of plasmas and flames. The C2 spectrum is characterized by many band systems with new ones still being actively identified; astronomical observations involve eight of these bands. Recently, Furtenbacher et al. (2016, Astrophys. J. Suppl., 224, 44) presented a set of 5699 empirical energy levels for 12C2, distributed among 11 electronic states and 98 vibronic bands, derived from 42 experimental studies and obtained using the MARVEL (Measured Active Rotational-Vibrational Energy Levels) procedure. Here, we add data from 13 new sources and update data from 5 sources. Many of these data sources characterize high-lying electronic states, including the newly detected 3Pig state. Older studies have been included following improvements in the MARVEL procedure which allow their uncertainties to be estimated. These older works in particular determine levels in the C1Pig state, the upper state of the insufficiently characterized Deslandres-d'Azambuja (C1Pig-A1Piu) band. The new compilation considers a total of 31323 transitions and derives 7047 empirical(MARVEL) energy levels spanning 20 electronic and 142 vibronic states. These new empirical energy levels are used here to update the 8states C2 ExoMol line list. This updated line list is highly suitable for high-resolution cross-correlation studies in astronomical spectroscopy of, for example, exoplanets, as 99.4% of the transitions with intensities over 10^(-18) cm/molecule at 1000 K have frequencies determined by empirical energy levels.

astro-ph.EP↗

arXiv2020

On the Computation of Dipole Moments: A Recommendation on the Choice of the Basis Set and the Level of Theory

Together with experimental data, theoretically predicted dipole moments represent a valuable tool for different branches in the chemical and physical sciences. With the diversity of levels of theory and basis sets available, a reliable combination must be carefully chosen in order to achieve accurate predictions. In a recent publication (arXiv:1709.05075 [physics.chem-ph]), Hait and Head-Gordon took a first step in this regard by providing recommendations on the best density functionals suitable for these purposes. However, no extensive study has been performed to provide recommendations on the basis set choice. Here, we shed some light into this matter by evaluating the performance of 38 general-purpose basis sets of single up to triple zeta-quality, when coupled with nine different levels of theory, in the computation of dipole moments. The calculations were performed on a data set with 114 small molecules containing second- and third-row elements. We based our analysis in regularised root mean square errors (regularised RMSE) as this procedure ensures relative errors for ionic species and absolute errors for species with small dipole moment values. Our results indicate that the best compromise between accuracy and computational efficiency is achieved by performing the computations with an augmented double zeta-quality basis set (i.e. aug-pc-1, aug-pcseg-1, aug-cc-pVDZ) together with a hybrid functional (e.g. wB97X-V, SOGGA11-X). Augmented triple-zeta basis sets could enhance the accuracy of the computations, but the computational cost of introducing such a basis set is substantial compared with the small improvement provided. These findings also highlight the crucial role that augmentation of the basis set with diffuse functions on both hydrogen and non-hydrogen atoms plays in the computation of dipole moments.

physics.chem-ph↗

arXiv2020

Experimental energy levels of 12C14N through MARVEL analysis

The cyano radical (CN) is a key molecule across many different factions of astronomy and chemistry. Accurate, empirical rovibronic energy levels with uncertainties are determined for 8 doublet states of CN using the \Marvel{} (Measured Active Rotational-Vibrational Energy Levels) algorithm. \notrans{} transitions were validated from \nosources{} different published sources to generate \noenergy{} spin-rovibronic energy levels. The empirical energy levels obtained from the \Marvel{} analysis are compared to current energy levels from the \Mollist{} line list. The \Mollist{} transition frequencies are updated with \Marvel{} energy level data which brings the frequencies obtained through experimental data up to \alert{77.3\%} from the original 11.3\%, with 92.6\% of the transitions with intensities over 10$^{-23}$ cm/molecule at 1000 K now known from experimental data. At 2000 K, 100.0\% of the partition function is recovered using only \Marvel{} energy levels, while 98.2\% is still recovered at 5000 K.

astro-ph.SR↗

arXiv2020

Computational insight into diatomic molecules as probes to measure the variation of the proton-to-electron mass ratio

Astrophysical molecular spectroscopy is an important means of searching for new physics through probing the variation of the proton-to-electron mass ratio, $μ$. New molecular probes could provide tighter constraints on the variation of $μ$ and better direction for theories of new physics. Here we summarise our previous paper \citep{19SyMoCu.CN} for astronomers, highlighting the importance of accurate estimates of peak molecular abundance and temperature as well as spectral resolution and sensitivity of telescopes in different regions of the electromagnetic spectrum. Whilst none of the 11 astrophysical diatomic molecules we investigated showed enhanced sensitive rovibronic transitions at observable intensities for astrophysical environments, we have gained a better understanding of the factors that contribute to high sensitivities. From our results, CN, CP, SiN and SiC have shown the most promise of all astrophysical diatomic molecules for further investigation, with further work currently being done on CN.

physics.atom-ph↗