Coupled cluster

About: Coupled cluster is a research topic. Over the lifetime, 6280 publications have been published within this topic receiving 301055 citations.

The accuracy of current density functionals for the calculation of electric field gradients: A comparison with ab initio methods for HCl and CuCl

Abstract: The performance of current density functionals is analyzed in detail for the electric field gradients (EFG) of hydrogen chloride and copper chloride by comparison with ab initio methods and available experimental data. The range of density functionals applied shows good agreement with coupled cluster H and Cl field gradients for HCl, as has been demonstrated previously for other main-group element containing compounds. However, the performance of most density functionals is very poor for the Cu EFG in CuCl (EFG for Cu -0.44 a.u. at the coupled-cluster singles and doubles with perturbative triples [CCSD(T)] level, compared to, e.g., +0.54 a.u. at the B-LYP level). Only the “half-and-half” hybrid functionals give field gradients with the correct sign. The reason for the poor performance of the density functional theory is analyzed in detail comparing density functional with ab initio total electronic densities ρ(r). Due to the conservation of the number of particles, a change in the valence part of the elec...

Heats of formation of xenon fluorides and the fluxionality of XeF(6) from high level electronic structure calculations.

Abstract: Atomization energies at 0 K and heats of formation at 0 and 298 K are predicted for XeF(+), XeF(-), XeF(2), XeF(4), XeF(5)(-), and XeF(6) from coupled cluster theory (CCSD(T)) calculations with new correlation-consistent basis sets for Xe. To achieve near chemical accuracy (+/-1 kcal/mol), up to four corrections were added to the complete basis set binding energies based on frozen core coupled cluster theory energies: a correction for core-valence effects, a correction for scalar relativistic effects, a correction for first-order atomic spin-orbit effects, and in some cases, a second-order spin-orbit correction. Vibrational zero-point energies were computed at the coupled cluster level of theory. The structure of XeF(6) is difficult to obtain with the C(3)(v)() and O(h)() structures having essentially the same energy. The O(h)() structure is only 0.19 kcal/mol below the C(3)(v)() one at the CCSD(T)/CBS level using an approximate geometry for the C(3)(v)() structure. With an optimized C(3)(v)() geometry, the C(3)(v)() structure would probably become slightly lower in energy than the O(h)() one. The calculated heats of formation for the neutral XeF(n)() fluorides are less negative than the experimental values from the equilibrium measurements by 2.0, 7.7, and 12.2 kcal/mol for n = 2, 4, and 6, respectively. For the experimental values, derived from the photoionization measurements, this discrepancy becomes even larger, suggesting a need for a redetermination of the experimental values. Evidence is presented for the fluxionality of XeF(6) caused by the presence of a sterically active, free valence electron pair on Xe.

Benchmark calculations on the electron detachment energies of MO3* and M2O6* (M = Cr, Mo, W).

Abstract: Neutral and anionic molecules of the monomers and dimers of the group VIB transition metal oxides (MO3 and M2O6) were studied with density functional theory (DFT) and coupled cluster CCSD(T) theory. Franck−Condon simulations of the photoelectron spectra were carried out for the transition from the ground state of the anion to that of the neutral molecule. Molecular structures from the DFT and CCSD(T) methods are compared. Electron detachment energies reported in the literature were evaluated. The calculated adiabatic and vertical electron detachment energies (ADEs and VDEs) were compared with the experimental results. CCSD(T) gives results within 0.12 eV for the ADEs. CCSD(T) predicts VDEs that are in error by as much as 0.3 eV for M = Cr. DFT hybrid functionals were found to give poor results for the ADEs and VDEs for M = Cr due to the substantial amount of multireference character in the wavefunction, whereas the pure DFT functionals give superior results. For M = Mo and W, excellent agreement was found...

Coupled-Cluster Valence-Bond Singles and Doubles for Strongly Correlated Systems: Block-Tensor Based Implementation and Application to Oligoacenes.

Abstract: We demonstrate a block-tensor based implementation of coupled-cluster valence-bond singles and doubles (CCVB-SD) [Small, D. W.; Head-Gordon M. J. Chem. Phys. 2012, 137, 114103] which is a simple modification to restricted CCSD (RCCSD) that provides a qualitatively correct description of valence correlations even in strongly correlated systems. We derive the Λ-equation of CCVB-SD and the corresponding unrelaxed density matrices. The resulting production-level implementation is applied to oligoacenes, correlating up to 318 electrons in 318 orbitals. CCVB-SD shows a qualitative agreement with exact methods for short acenes and reaches the bulk limit of oligoacenes in terms of natural orbital occupation numbers, whereas RCCSD shows nonvariational behavior even for relatively short acenes. A significant reduction in polyradicaloid character is found when correlating all valence electrons instead of only the π-electrons.

Origin of the SN2 benzylic effect.

Abstract: The SN2 identity exchange reactions of the fluoride ion with benzyl fluoride and 10 para-substituted derivatives (RC6H4CH2F, R = CH3, OH, OCH3, NH2, F, Cl, CCH, CN, COF, and NO2) have been investigated by both rigorous ab initio methods and carefully calibrated density functional theory. Groundbreaking focal-point computations were executed for the C6H5CH2F + F− and C6H5CH2Cl + Cl− SN2 reactions at the highest possible levels of electronic structure theory, employing complete basis set (CBS) extrapolations of aug-cc-pVXZ (X = 2−5) Hartree−Fock and MP2 energies, and including higher-order electron correlation via CCSD/aug-cc-pVQZ and CCSD(T)/aug-cc-pVTZ coupled cluster wave functions. Strong linear dependences are found between the computed electrostatic potential at the reaction-center carbon atom and the effective SN2 activation energies within the series of para-substituted benzyl fluorides. An activation strain energy decomposition indicates that the SN2 reactivity of these benzylic compounds is govern...