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1. Reliable Structures and Electronic Energies of Small Water Clusters Using Density Functional and Local Correlation Coupled Cluster Model Chemistries

   https://doi.org/10.1021/acs.jpca.5c04923  

   Petty, Benjamin T; Fowler, Vance R; Ryu, Audrey; Glick, Caroline S; Rock, Carly A; Wang, Qihang; Tschumper, Gregory S; Shields, George C (September 2025, The Journal of Physical Chemistry A)

   Not AvailableIn this paper we have assessed the ability of the domain-based local pair natural orbital (DLPNO)-CCSD(T) method to match the explicitly correlated CCSD(T) relative energies of (H2O)n=3−7 isomers along with the impact of the level of theory utilized to optimize the water cluster geometries. The benchmark structures were optimized using a 2-body:Many-body procedure in which all of the 1- and 2-body contributions are computed using CCSD(T) while all of the higher-order many-body interactions are computed using MP2 (denoted CCSD(T):MP2). Benchmark relative energies were computed for these CCSD-(T):MP2 optimized geometries with explicitly correlated CCSD- (T)-F12b single point energies (SPEs) using the cc-pVQZ-F12 and cc-pV5Z-F12 basis sets augmented with diffuse functions on the O atoms. The benchmark structures and energies were used to gauge the performance of less demanding computational protocols. For example, DLPNO−CCSD(T) computations on the 31 benchmark structures with the analogous family of correlation consistent basis sets (cc-pVNZ for H and aug-cc-pVNZ for O, or simply haNZ where N = D-6) were used to estimate relative energies at the complete basis set (CBS) limit via three-point extrapolations. When compared to the CCSD(T)-F12 benchmark data, the mean absolute differences (MADs) were ≤ 0.13 kcal/mol when triple-ζ and larger basis sets were employed. Using these DLPNO−CCSD(T) results, we demonstrate that 2 less-demanding geometry optimization procedures, specifically the ωB97X-D density functional theory (DFT) method paired with the 6−31++G(d,p) basis set and the density-fitted MP2 method paired with the haTZ basis set, give structures that yield nearly identical relative energies (MADs of only 0.07 and 0.02 kcal/mol, respectively, when comparing DLPNO−CCSD(T)/ha6Z data). In addition, we show how the presence or absence of diffuse functions in the basis sets used for DLPNO−CCSD(T) SPEs impact the quality of the relative energies. The protocol that combines ωB97X-D/6−31++G(d,p) optimized structures with DLPNO−CCSD(T) SPEs using triple-ζ or higher Dunning basis sets that include augmentation with diffuse functions on the oxygen atoms provides a fast and accurate method for determining the relative electronic energies of (H2O)n=3−7 water cluster isomers. 

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2. Semi-experimental equilibrium ( r e SE) and theoretical structures of hydrazoic acid (HN3)

   https://doi.org/10.1063/5.0101064  

   Owen, Andrew N.; Sahoo, Nitai P.; Esselman, Brian J.; Stanton, John F.; Woods, R. Claude; McMahon, Robert J. (July 2022, The Journal of Chemical Physics)

   Hydrazoic acid (HN3) is used as a case study for investigating the accuracy and precision by which a molecular structure—specifically, a semi-experimental equilibrium structure (reSE)—may be determined using current state-of-the-art methodology. The influence of the theoretical corrections for effects of vibration–rotation coupling and electron-mass distribution that are employed in the analysis is explored in detail. The small size of HN3 allowed us to deploy considerable computational resources to probe the basis-set dependence of these corrections using a series of coupled-cluster single, double, perturbative triple [CCSD(T)] calculations with cc-pCVXZ (X = D, T, Q, 5) basis sets. We extrapolated the resulting corrections to the complete basis set (CBS) limit to obtain CCSD(T)/CBS corrections, which were used in a subsequent reSE structure determination. The reSE parameters obtained using the CCSD(T)/cc-pCV5Z corrections are nearly identical to those obtained using the CCSD(T)/CBS corrections, with uncertainties in the bond distances and angles of less than 0.0006 Å and 0.08°, respectively. The previously obtained reSE structure using CCSD(T)/ANO2 agrees with that using CCSD(T)/cc-pCV5Z to within 0.000 08 Å and 0.016° for bond distances and angles, respectively, and with only 25% larger uncertainties, validating the idea that reSE structure determinations can be carried out with significantly smaller basis sets than those needed for similarly accurate, strictly ab initio determinations. Although the purely computational re structural parameters [CCSD(T)/cc-pCV6Z] fall outside of the statistical uncertainties (2σ) of the corresponding reSE structural parameters, the discrepancy is rectified by applying corrections to address the theoretical limitations of the CCSD(T)/cc-pCV6Z geometry with respect to basis set, electron correlation, relativity, and the Born–Oppenheimer approximation, thereby supporting the contention that the semi-experimental approach is both an accurate and vastly more efficient method for structure determinations than is brute-force computation. 

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3. 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. 

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4. Insight into the Binding of Argon to Cyclic Water Clusters from Symmetry-Adapted Perturbation Theory

   https://doi.org/10.3390/ijms242417480  

   Rock, Carly A.; Tschumper, Gregory S. (December 2023, International Journal of Molecular Sciences)

   This work systematically examines the interactions between a single argon atom and the edges and faces of cyclic H2O clusters containing three–five water molecules (Ar(H2O)n=3–5). Full geometry optimizations and subsequent harmonic vibrational frequency computations were performed using MP2 with a triple-ζ correlation consistent basis set augmented with diffuse functions on the heavy atoms (cc-pVTZ for H and aug-cc-pVTZ for O and Ar; denoted as haTZ). Optimized structures and harmonic vibrational frequencies were also obtained with the two-body–many-body (2b:Mb) and three-body–many-body (3b:Mb) techniques; here, high-level CCSD(T) computations capture up through the two-body or three-body contributions from the many-body expansion, respectively, while less demanding MP2 computations recover all higher-order contributions. Five unique stationary points have been identified in which Ar binds to the cyclic water trimer, along with four for (H2O)4 and three for (H2O)5. To the best of our knowledge, eleven of these twelve structures have been characterized here for the first time. Ar consistently binds more strongly to the faces than the edges of the cyclic (H2O)n clusters, by as much as a factor of two. The 3b:Mb electronic energies computed with the haTZ basis set indicate that Ar binds to the faces of the water clusters by at least 3 kJ mol−1 and by nearly 6 kJ mol−1 for one Ar(H2O)5 complex. An analysis of the interaction energies for the different binding motifs based on symmetry-adapted perturbation theory (SAPT) indicates that dispersion interactions are primarily responsible for the observed trends. The binding of a single Ar atom to a face of these cyclic water clusters can induce perturbations to the harmonic vibrational frequencies on the order of 5 cm−1 for some hydrogen-bonded OH stretching frequencies. 

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5. Optimization of damping function parameters for -D3 and -D4 dispersion models for Hartree–Fock based symmetry-adapted perturbation theory

   https://doi.org/10.1063/5.0219185  

   Wallace, Austin M; Sherrill, C David (September 2024, The Journal of Chemical Physics)

   Symmetry-adapted perturbation theory (SAPT) directly computes intermolecular interaction energy in terms of electrostatics, exchange-repulsion, induction/polarization, and London dispersion components. In SAPT based on Hartree–Fock (“SAPT0”) or based on density functional theory, the most time-consuming step is the computation of the dispersion terms. Previous work has explored the replacement of these expensive dispersion terms with simple damped asymptotic models. We recently examined [Schriber et al. J. Chem. Phys. 154, 234107 (2021)] the accuracy of SAPT0 when replacing its dispersion term with Grimme’s popular -D3 correction, reducing the computational cost scaling from O(N5) to O(N3). That work optimized damping function parameters for SAPT0-D3/jun-cc-pVDZ using estimates of the coupled-cluster complete basis set limit [CCSD(T)/CBS] on a 8299 dimer dataset. Here, we explore the accuracy of SAPT0-D3 with additional basis sets, along with an analogous model using -D4. Damping parameters are rather insensitive to basis sets, and the resulting SAPT0-D models are more accurate on average for total interaction energies than SAPT0. Our results are surprising in several respects: (1) improvement of -D4 over -D3 is negligible for these systems, even charged systems where -D4 should, in principle, be more accurate; (2) addition of Axilrod–Teller–Muto terms for three-body dispersion does not improve error statistics for this test set; and (3) SAPT0-D is even more accurate on average for total interaction energies than the much more computationally costly density functional theory based SAPT [SAPT(DFT)] in an aug-cc-pVDZ basis. However, SAPT0 and SAPT0-D3/D4 interaction energies benefit from significant error cancellation between exchange and dispersion terms. 

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