Topic
Coupled cluster
About: Coupled cluster is a research topic. Over the lifetime, 6280 publications have been published within this topic receiving 301055 citations.
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TL;DR: In this paper, a multireference coupled electron-pair method (MR CEPM) is presented and applied in pilot calculations for quasidegenerate states, where it is assumed that the individual cluster operators can be approximated by their two body parts.
Abstract: A size-extensive, multireferences coupled-cluster method for studies of quasidegenerate states based on the Jeziorski–Monkhorst [16] ansatz for the cluster operator (Ω = ∑ePj, where the sum is extended over the configurations spanning the model space), is presented and applied in pilot calculations. The method is referred to as multireference coupled electron-pair method (MR CEPM), because it is assumed that the individual cluster operators can be approximated by their two-body parts, i.e., Tj ≈ Tj(2). The linear version of this method (MR L-CEPM) is also discussed. Both methods are applied to two simple model systems: (1) a minimum basis set model involving eight hydrogen atoms in various spacial arrangements for which the degree of quasidegeneracy can be continuously varied; (2) a model involving the C2ν insertion of Be into H2. For the first time in multireference coupled-cluster calculations, the nonlinear parts of the equations are completely accounted for. The MR CEPM results are very encouraging for strongly quasidegenerate states. The MR L-CEPM results are slightly below the accurate (FCI) values.
150 citations
TL;DR: In this paper, an augmented coupled cluster scheme was proposed to evaluate the higher order electron correlation effects, which is correct to fourth order in a perturbation expansion and includes significant fifth and higher order terms.
Abstract: An augmented coupled cluster scheme to evaluate the higher order electron correlation effects is proposed. The method is carried out in two steps. First, a coupled cluster calculation with all double substitutions (CCD) is performed. The converged CCD wave function is then used in the evaluation of the contribution of single and triple substitutions. The method is correct to fourth order in a perturbation expansion and includes significant fifth and higher order terms. Illustrative calculations on the excitation and dissociation energies of first‐row homonuclear diatomic molecules are reported. The low‐lying excitation energies of B2 and C2 are accurately calculated. The dissociation energies of B2, C2, N2, O2, and F2 are all uniformly underestimated by 0.1–0.3 eV using large spdf basis sets.
150 citations
TL;DR: The results of the present study emphasize the importance of long-range exchange corrections in TDDFT for investigating the excited-state properties in solar cell dyes and find that the LC technique provides a consistent picture of charge-transfer excitations as a function of molecular size.
Abstract: The excited-state properties in a series of coumarin solar cell dyes are investigated with a long-range-corrected (LC) functional which asymptotically incorporates Hartree-Fock exchange. Using time-dependent density functional theory (TDDFT), we calculate excitation energies, oscillator strengths, and excited-state dipole moments in each of the dyes as a function of the range-separation paramenter, mu. To investigate the acceptable range of mu and assess the quality of the LC-TDDFT formalism, an extensive comparison is made between LC-BLYP excitation energies and approximate coupled cluster singles and doubles (CC2) calculations. When using a properly-optimized value of mu, we find that the LC technique provides a consistent picture of charge-transfer excitations as a function of molecular size. In contrast, we find that the widely-used B3LYP hybrid functional severely overestimates excited-state dipole moments and underestimates vertical excitations energies, especially for larger dye molecules. The results of the present study emphasize the importance of long-range exchange corrections in TDDFT for investigating the excited-state properties in solar cell dyes.
150 citations
TL;DR: Complementary ab initio QM/MM molecular dynamics simulations with umbrella sampling have been carried out to determine the free energy profiles of zwitterion formation and proton transfer in solution; the results confirm that the formation of the zwittersion is rate-determining.
Abstract: We present a theoretical study of the reaction mechanism of monoethanolamine (MEA) with CO2 in an aqueous solution. We have used molecular orbital reaction pathway calculations to compute reaction free energy landscapes for the reaction steps involved in the formation of carbamic acids and carbamates. We have used the conductor-like polarizable continuum model to calculate reactant, product, and transition state geometries and vibrational frequencies within density functional theory (DFT). We have also computed single point energies for all stationary structures using a coupled cluster approach with singles, doubles, and perturbational triple excitations using the DFT geometries. Our calculations indicate that a two-step reaction mechanism that proceeds via a zwitterion intermediate to form carbamate is the most favorable reaction channel. The first step, leading to formation of the zwitterion, is found to be rate-determining, and the activation free energies are 12.0 (10.2) and 11.3 (9.6) kcal/mol using ...
150 citations
TL;DR: In this article, the Moller-Plesset perturbation theory of the second order (MP2) was employed to compute the polycyclic aromatic hydrocarbon (PAH)-water interaction energies, with proper size of basis sets and energy component analysis to extrapolate to infinite-sized graphene limit.
Abstract: In this study, we develop graphitic carbon-water nonbonded interaction parameters entirely from ab initio calculation data of interaction energies between graphene and a single water molecule. First, we employ the Moller-Plesset perturbation theory of the second order (MP2) method to compute the polycyclic aromatic hydrocarbon (PAH)-water interaction energies, with proper size of basis sets and energy component analysis to extrapolate to infinite-sized graphene limit. Then, we develop graphitic carbon-water interaction parameters based on the MP2 data from this work and the ab initio data available in the literature from other methods such as random-phase approximation (RPA), density functional theory-symmetry-adapted perturbation theory (DFT-SAPT), and coupled cluster treatment with single and double excitations and perturbative triples (CCSD(T)). The accuracy of the interaction parameters is evaluated by predicting water contact angle on graphite and carbon nanotube (CNT) radial breathing mode (RBM) frequency shift and comparing them with experimental data. The interaction parameters obtained from MP2 data predict the CNT RBM frequency shift that is in good agreement with experiments. The interaction parameters obtained from RPA and DFT-SAPT data predict the contact angles and the CNT RBM frequency shift that agree well with experiments. The interaction parameters obtained from CCSD(T) data underestimate the contact angles and overestimate the CNT RBM frequency shift probably due to the use of small basis sets in CCSD(T) calculations.
149 citations