More Like this

1. Conformational comparison of urea and thiourea near the CCSD ( T ) complete basis set limit

   https://doi.org/10.1002/qua.27075  

   Barlow, Kayleigh R.; Tschumper, Gregory S. (December 2022, International Journal of Quantum Chemistry)

   Abstract The global minima of urea and thiourea were characterized along with other low‐lying stationary points. Each structure was optimized with the CCSD(T) method and triple‐ζcorrelation consistent basis sets followed by harmonic vibrational frequency computations. Relative energies evaluated near the complete basis set limit with both canonical and explicitly correlated CCSD(T) techniques reveal several subtle but important details about both systems. These computations resolve a discrepancy by demonstrating that the electronic energy of the C_2vsecond‐order saddle point of urea lies at least 1.5 kcal mol⁻¹above the C₂global minimum regardless of whether the structures were optimized with MP2, CCSD, or CCSD(T). Additionally, urea effectively has one minimum instead of two because the electronic barrier for inversion at one amino group in the C_slocal minimum vanishes at the CCSD(T) CBS limit. Characterization of both systems with the same ab initio methods and large basis sets conclusively establishes that the electronic barriers to inversion at one or both NH₂groups in thiourea are appreciably smaller than in urea. CCSDT(Q)/cc‐pVTZ computations show higher‐order electron correlation effects have little impact on the relative energies and are consistently offset by core correlation effects of opposite sign and comparable magnitude. 

   more » « less
2. The HOX⋯SO ₂ (X=F, Cl, Br, I) Binary Complexes: Implications for Atmospheric Chemistry

   https://doi.org/10.1002/cphc.202000746  

   Kitzmiller, Nathaniel L.; Wolf, Mark E.; Turney, Justin M.; Schaefer, III, Henry F. (December 2020, ChemPhysChem)

   Abstract Sulfur dioxide and hypohalous acids (HOX, X=F, Cl, Br, I) are ubiquitous molecules in the atmosphere that are central to important processes like seasonal ozone depletion, acid rain, and cloud nucleation. We present the first theoretical examination of the HOX⋯SO₂binary complexes and the associated trends due to halogen substitution. Reliable geometries were optimized at the CCSD(T)/aug‐cc‐pV(T+d)Z level of theory for HOF and HOCl complexes. The HOBr and HOI complexes were optimized at the CCSD(T)/aug‐cc‐pV(D+d)Z level of theory with the exception of the Br and I atoms which were modeled with an aug‐cc‐pwCVDZ‐PP pseudopotential. 27 HOX⋯SO₂complexes were characterized and the focal point method was employed to produce CCSDT(Q)/CBS interaction energies. Natural Bond Orbital analysis and Symmetry Adapted Perturbation Theory were used to classify the nature of each principle interaction. The interaction energies of all HOX⋯SO₂complexes in this study ranged from 1.35 to 3.81 kcal mol⁻¹. The single‐interaction hydrogen bonded complexes spanned a range of 2.62 to 3.07 kcal mol⁻¹, while the single‐interaction halogen bonded complexes were far more sensitive to halogen substitution ranging from 1.35 to 3.06 kcal mol⁻¹, indicating that the two types of interactions are extremely competitive for heavier halogens. Our results provide insight into the interactions between HOX and SO₂which may guide further research of related systems. 

   more » « less
3. Examination of the structures, energetics, and vibrational frequencies of small sulfur‐containing prototypical dimers, (H ₂ S) ₂ and H ₂ O/H ₂ S

   https://doi.org/10.1002/jcc.25578  

   Dreux, Katelyn M.; Tschumper, Gregory S. (October 2018, Journal of Computational Chemistry)

   The optimized geometries, vibrational frequencies, and dissociation energies from MP2 and CCSD(T) computations with large correlation consistent basis sets are reported for (H₂S)₂and H₂O/H₂S. Anharmonic vibrational frequencies have also been computed with second‐order vibrational perturbation theory (VPT2). As such, the fundamental frequencies, overtones, and combination bands reported in this study should also provide a useful road map for future spectroscopic studies of the simple but important heterogeneous H₂O/H₂S dimer in which the hydrogen bond donor and acceptor can interchange, leading to two unique minima with very similar energies. Near the CCSD(T) complete basis set limit, the HOH⋯SH₂configuration (H₂O donor) lies only 0.2 kcal mol⁻¹below the HSH⋯OH₂structure (H₂S donor). When the zero‐point vibrational energy is included, however, the latter configuration becomes slightly lower in energy than the former by <0.1 kcal mol⁻¹. © 2018 Wiley Periodicals, Inc. 

   more » « less
4. A comparison between hydrogen and halogen bonding: the hypohalous acid–water dimers, HOX⋯H ₂ O (X = F, Cl, Br)

   https://doi.org/10.1039/C9CP00422J  

   Wolf, Mark E.; Zhang, Boyi; Turney, Justin M.; Schaefer, Henry F. (March 2019, Physical Chemistry Chemical Physics)

   Hypohalous acids (HOX) are a class of molecules that play a key role in the atmospheric seasonal depletion of ozone and have the ability to form both hydrogen and halogen bonds. The interactions between the HOX monomers (X = F, Cl, Br) and water have been studied at the CCSD(T)/aug-cc-pVTZ level of theory with the spin free X2C-1e method to account for scalar relativistic effects. Focal point analysis was used to determine CCSDT(Q)/CBS dissociation energies. The anti hydrogen bonded dimers were found with interaction energies of −5.62 kcal mol −1 , −5.56 kcal mol −1 , and −4.97 kcal mol −1 for X = F, Cl, and Br, respectively. The weaker halogen bonded dimers were found to have interaction energies of −1.71 kcal mol −1 and −3.03 kcal mol −1 for X = Cl and Br, respectively. Natural bond orbital analysis and symmetry adapted perturbation theory were used to discern the nature of the halogen and hydrogen bonds and trends due to halogen substitution. The halogen bonds were determined to be weaker than the analogous hydrogen bonds in all cases but close enough in energy to be relevant, significantly more so with increasing halogen size. 

   more » « less
5. Benchmark coupled-cluster lattice energy of crystalline benzene and assessment of multi-level approximations in the many-body expansion

   https://doi.org/10.1063/5.0159410  

   Borca, Carlos H.; Glick, Zachary L.; Metcalf, Derek P.; Burns, Lori A.; Sherrill, C. David (June 2023, The Journal of Chemical Physics)

   The many-body expansion (MBE) is promising for the efficient, parallel computation of lattice energies in organic crystals. Very high accuracy should be achievable by employing coupled-cluster singles, doubles, and perturbative triples at the complete basis set limit [CCSD(T)/CBS] for the dimers, trimers, and potentially tetramers resulting from the MBE, but such a brute-force approach seems impractical for crystals of all but the smallest molecules. Here, we investigate hybrid or multi-level approaches that employ CCSD(T)/CBS only for the closest dimers and trimers and utilize much faster methods like Møller–Plesset perturbation theory (MP2) for more distant dimers and trimers. For trimers, MP2 is supplemented with the Axilrod–Teller–Muto (ATM) model of three-body dispersion. MP2(+ATM) is shown to be a very effective replacement for CCSD(T)/CBS for all but the closest dimers and trimers. A limited investigation of tetramers using CCSD(T)/CBS suggests that the four-body contribution is entirely negligible. The large set of CCSD(T)/CBS dimer and trimer data should be valuable in benchmarking approximate methods for molecular crystals and allows us to see that a literature estimate of the core-valence contribution of the closest dimers to the lattice energy using just MP2 was overbinding by 0.5 kJ mol−1, and an estimate of the three-body contribution from the closest trimers using the T0 approximation in local CCSD(T) was underbinding by 0.7 kJ mol−1. Our CCSD(T)/CBS best estimate of the 0 K lattice energy is −54.01 kJ mol−1, compared to an estimated experimental value of −55.3 ± 2.2 kJ mol−1. 

   more » « less