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1. Prospects for rank-reduced CCSD(T) in the context of high-accuracy thermochemistry

   https://doi.org/10.1063/5.0230899  

   Zhao, Tingting; Thorpe, James H; Matthews, Devin A (October 2024, The Journal of Chemical Physics)

   Obtaining sub-chemical accuracy (1 kJ mol−1) for reaction energies of medium-sized gas-phase molecules is a longstanding challenge in the field of thermochemical modeling. The perturbative triples correction to coupled-cluster single double triple [CCSD(T)] constitutes an important component of all high-accuracy composite model chemistries that obtain this accuracy but can be a roadblock in the calculation of medium to large systems due to its O(N7) scaling, particularly in HEAT-like model chemistries that eschew separation of core and valence correlation. This study extends the work of Lesiuk [J. Chem. Phys. 156, 064103 (2022)] with new approximate methods and assesses the accuracy of five different approximations of (T) in the context of a subset of molecules selected from the W4-17 dataset. It is demonstrated that all of these approximate methods can achieve sub-0.1 kJ mol−1 accuracy with respect to canonical, density-fitted (T) contributions with a modest number of projectors. The approximation labeled Z̃T appears to offer the best trade-off between cost and accuracy and shows significant promise in an order-of-magnitude reduction in the computational cost of the CCSD(T) component of high-accuracy model chemistries. 

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2. Factorized Quadruples and a Predictor of Higher-Level Correlation in Thermochemistry

   https://doi.org/10.1021/acs.jpca.4c04460  

   Thorpe, James H; Windom, Zachary W; Bartlett, Rodney J; Matthews, Devin A (September 2024, The Journal of Physical Chemistry A)

   Coupled cluster theory has had a momentous impact on the ab initio prediction of molecular properties, and remains a staple ingratiate in high-accuracy thermochemical model chemistries. However, these methods require inclusion of at least some connected quadruple excitations, which generally scale at best as 𝒪(𝑁9) with the number of basis functions. It is very difficult to predict, a priori, the effect correlation of past CCSD(T) on a given reaction energy. The purpose of this work is to examine cost-effective quadruple corrections based on the factorization theorem of the many-body perturbation theory that may address these challenges. We show that the 𝒪(𝑁7) factorized CCSD(TQf) method introduces minimal error to predicted correlation and reaction energies as compared to the 𝒪(𝑁9) CCSD(TQ). Further, we examine the performance of Goodson’s continued fraction method in the estimation of CCSDT(Q)Λ contributions to reaction energies as well as a “new” method related to %TAE[(T)] that we refer to as a scaled perturbation estimator. We find that the scaled perturbation estimator based upon CCSD(TQf)/cc-pVDZ is capable of predicting CCSDT(Q)Λ/cc-pVDZ contributions to reaction energies with an average error of 0.07 kcal mol–1 and an L2D of 0.52 kcal mol–1 when applied to a test-suite of nearly 3000 reactions. This offers a means by which to reliably “ballpark” how important post-CCSD(T) contributions are to reaction energies while incurring no more than CCSD(T) formal cost and a little mental math. 

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3. Scalable Ab Initio Electronic Structure Methods with Near Chemical Accuracy for Main Group Chemistry

   https://doi.org/10.1021/acs.jpca.4c02853  

   Wei, Yujing; Debnath, Sibali; Weber, John L; Mahajan, Ankit; Reichman, David R; Friesner, Richard A (July 2024, The Journal of Physical Chemistry A)

   This study evaluates the precision of widely recognized quantum chemical methodologies, CCSD(T), DLPNO−CCSD(T), and localized ph-AFQMC, for determining the thermochemistry of main group elements. DLPNO−CCSD- (T) and localized ph-AFQMC, which o(er greater scalability compared to canonical CCSD(T), have emerged over the past decade as pivotal in producing precise benchmark chemical data. Our investigation includes closed-shell, neutral molecules, focusing on their heat of formation and atomization energy sourced from four specific small molecule data sets. First, we selected molecules from the G2 and G3 data sets, noted for their reliable experimental heat of formation data. Additionally, we incorporate molecules from the W4−11 and W4−17 sets, which provide high-level theoretical reference values for atomization energy at 0 K. Our findings reveal that both DLPNO−CCSD(T) and ph-AFQMC methods are capable of achieving a root-mean-square deviation of less than 1 kcal/mol across the combined data set, aligning with the threshold for chemical accuracy. Moreover, we make e(orts to confine the maximum deviations within 2 kcal/mol, a degree of precision that significantly broadens the applicability of these methods in fields such as biology and materials science. 

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4. Approximating large-basis coupled-cluster theory vibrational frequencies using focal-point approximations

   https://doi.org/10.1063/5.0168608  

   Nelson, Philip M; Glick, Zachary L; Sherrill, C David (September 2023, The Journal of Chemical Physics)

   The focal-point approximation can be used to estimate a high-accuracy, slow quantum chemistry computation by combining several lower-accuracy, faster computations. We examine the performance of focal-point methods by combining second-order Møller–Plesset perturbation theory (MP2) with coupled-cluster singles, doubles, and perturbative triples [CCSD(T)] for the calculation of harmonic frequencies and that of fundamental frequencies using second-order vibrational perturbation theory (VPT2). In contrast to standard CCSD(T), the focal-point CCSD(T) method approaches the complete basis set (CBS) limit with only triple-ζ basis sets for the coupled-cluster portion of the computation. The predicted harmonic and fundamental frequencies were compared with the experimental values for a set of 20 molecules containing up to six atoms. The focal-point method combining CCSD(T)/aug-cc-pV(T + d)Z with CBS-extrapolated MP2 has mean absolute errors vs experiment of only 7.3 cm−1 for the fundamental frequencies, which are essentially the same as the mean absolute error for CCSD(T) extrapolated to the CBS limit using the aug-cc-pV(Q + d)Z and aug-cc-pV(5 + d)Z basis sets. However, for H2O, the focal-point procedure requires only 3% of the computation time as the extrapolated CCSD(T) result, and the cost savings will grow for larger molecules. 

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5. 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. 

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