Surprising performance for vibrational frequencies of the distinguishable clusters with singles and doubles (DCSD) and MP2.5 approximations (original) (raw)
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W4Λ: Leveraging Λ Coupled-Cluster for Accurate Computational Thermochemistry Approaches
Journal of Physical Chemistry A, 2024
High-accuracy composite wave function methods like Weizmann-4 (W4) theory, high-accuracy extrapolated ab initio thermochemistry (HEAT), and the Feller−Peterson−Dixon (FPD) approach enable sub-kJ/mol accuracy in gas-phase thermochemical properties. Their biggest computational bottleneck is the evaluation of the valence post-CCSD(T) correction term. We demonstrate here, for the W4-17 thermochemistry benchmark and subsets thereof, that the Λ coupled-cluster expansion converges more rapidly and smoothly than the regular coupled-cluster series. By means of CCSDT(Q) Λ and CCSDTQ(5) Λ , we can considerably (up to an order of magnitude) accelerate W4-and W4.3-type calculations without loss in accuracy, leading to the W4Λ and W4.3Λ computational thermochemistry protocols.
The Journal of Chemical Physics, 2008
There has been much interest in cost-free improvements to second-order Møller-Plesset perturbation theory ͑MP2͒ via scaling the same-and opposite-spin components of the correlation energy ͑spin-component scaled MP2͒. By scaling the same-and opposite-spin components of the double excitation correlation energy from the coupled-cluster of single and double excitations ͑CCSD͒ method, similar improvements can be achieved. Optimized for a set of 48 reaction energies, scaling factors were determined to be 1.13 and 1.27 for the same-and opposite-spin components, respectively. Preliminary results suggest that the spin-component scaled CCSD ͑SCS-CCSD͒ method will outperform all MP2 type methods considered for describing intermolecular interactions. Potential energy curves computed with the SCS-CCSD method for the sandwich benzene dimer and methane dimer reproduce the benchmark CCSD͑T͒ potential curves with errors of only a few hundredths of 1 kcal mol −1 for the minima. The performance of the SCS-CCSD method suggests that it is a reliable, lower cost alternative to the CCSD͑T͒ method.
WIREs Computational Molecular Science, 2019
While methodological developments in the last decade made it possible to compute coupled cluster (CC) energies including excitations up to a perturbative triples correction for molecules containing several hundred atoms, a similar breakthrough has not yet been reported for excited state computations. Accurate CC methods for excited states are still expensive, although some promising candidates for an efficient and accurate excited state CC method have emerged recently. This review examines the various approximation schemes with particular emphasis on their performance for excitation energies and summarizes the best state-of-the-art results which may pave the way for a robust excited state method applicable to molecules of hundreds of atoms. Among these, special attention will be given to exploiting the techniques of similarity transformation, perturbative approximations as well as integral decomposition, local and embedding techniques within the equation of motion CC framework.
Physical Review B, 2010
The mod-n scheme is introduced to the coupled-cluster singles and doubles ͑CCSD͒ and third-order Møller-Plesset perturbation ͑MP3͒ methods for extended systems of one-dimensional periodicity. By downsampling uniformly the wave vectors in Brillouin-zone integrations, this scheme accelerates these accurate but expensive correlation-energy calculations by two to three orders of magnitude while incurring negligible errors in their total and relative energies. To maintain this accuracy, the number of the nearest-neighbor unit cells included in the lattice sums must also be reduced by the same downsampling rate ͑n͒. The mod-n CCSD and MP3 methods are applied to the potential-energy surface of polyethylene in anharmonic frequency calculations of its infraredand Raman-active vibrations. The calculated frequencies are found to be within 46 cm −1 ͑CCSD͒ and 78 cm −1 ͑MP3͒ of the observed.
International Journal of Quantum Chemistry, 2019
Anharmonic vibrational frequencies for closed-shell molecules computed with CCSD(T)-F12b/augcc-pVTZ differ from significantly more costly composite energy methods by a mean absolute error (MAE) of 7.5 cm −1 per fundamental frequency. Comparison to a few available gas phase experimental modes, however, actually lowers the MAE to 6.0 cm −1. Open-shell molecules have an MAE of nearly a factor of six greater. Hence, open-shell molecular anharmonic frequencies cannot be as well-described with only explicitly correlated coupled cluster theory as their closed-shell brethren. As a result, the use of quartic force fields and vibrational perturbation theory can be opened to molecules with six or more atoms, whereas previously such computations were limited to molecules of five or fewer atoms. This will certainly assist in studies of more chemically interesting species, especially for atmospheric and interstellar infrared spectroscopic characterization.
Performance of coupled cluster theory in thermochemical calculations of small halogenated compounds
The Journal of Chemical Physics, 2003
Atomization energies at 0 K and heats of formation at 298 K were obtained for a collection of small halogenated molecules from coupled cluster theory including noniterative, quasiperturbative triple excitations calculations with large basis sets ͑up through augmented septuple zeta quality in some cases͒. In order to achieve near chemical accuracy ͑Ϯ1 kcal/mol͒ in the thermodynamic properties, we adopted a composite theoretical approach which incorporated estimated complete basis set binding energies based on frozen core coupled cluster theory energies and ͑up to͒ five corrections: ͑1͒ a core/valence correction; ͑2͒ a Douglas-Kroll-Hess scalar relativistic correction; ͑3͒ a first-order atomic spin-orbit correction; ͑4͒ a second-order spin-orbit correction for heavy elements; and ͑5͒ an approximate correction to account for the remaining correlation energy. The last of these corrections is based on a recently proposed approximation to full configuration interaction via a continued fraction approximant for coupled cluster theory ͓CCSD͑T͒-cf͔. Failure to consider corrections ͑1͒ to ͑4͒ can introduce errors significantly in excess of the target accuracy of Ϯ1 kcal/mol. Although some cancellation of error may occur if one or more of these corrections is omitted, such a situation is by no means universal and cannot be relied upon for high accuracy. The accuracy of the Douglas-Kroll-Hess approach was calibrated against both new and previously published four-component Dirac Coulomb results at the coupled cluster level of theory. In addition, vibrational zero-point energies were computed at the coupled cluster level of theory for those polyatomic systems lacking an experimental anharmonic value.
The Journal of Chemical Physics, 2003
Calculated vertical excitation energies, optimized geometries, and vibrational frequencies of the nitric oxide dimer are reported. The ''multireference'' ͑MR͒ nature of the problem and weak bond between the monomers make a proper description of the system difficult, and standard methods are not as applicable to this system. In this study, recently developed methods such as the double-electron-affinity similarity-transformed equation-of-motion coupled cluster method ͑DEA-STEOM-CCSD͒, MR Brillouin-Wigner CCSD ͑MR-BWCCSD͒, MR average quadratic CCSD ͑MR-AQCCSD͒, and others are used along with a series of basis sets of increasing accuracy. The calculated excitation energies are consistent and convergent with respect to the basis set in DEA-STEOM-CCSD, MR-BWCCSD, and MR-AQCCSD methods. The geometries are highly sensitive to the basis set size and the challenge to obtain the right answers in the basis set limit remains. Nevertheless, we obtain qualitative agreement with the experimental geometry and harmonic vibrational frequencies. The results from the above multireference methods show dramatic improvement over the coupled cluster with singles and doubles and perturbative triples excitation ͓CCSD͑T͔͒ results. Like O 3 , (NO) 2 offers an extremely challenging example in its ground and excited states for single-reference and multireference theory. It deserves to be a standard test molecule as new methods are developed.