Coupled cluster

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

Recursive intermediate factorization and complete computational linearization of the coupled-cluster single, double, triple, and quadruple excitation equations

Abstract: The nonlinear CCSDTQ equations are written in a fully linearized form, via the introduction of computationally convenient intermediates. An efficient formulation of the coupled cluster method is proposed. Due to a recursive method for the calculation of intermediates, all computational steps involve the multiplication of an intermediate with aT vertex. This property makes it possible to express the CC equations exclusively in terms of matrix products which can be directly transformed into a highly vectorized program.

A new method for excited states: Similarity transformed equation-of-motion coupled-cluster theory

Abstract: We present the first application of the similarity transformed equation-of-motion coupled-cluster method (STEOM-CC) to calculate excited states. STEOM-CC theory arises from a similarity transform of the second quantized Hamiltonian which strongly reduces the coupling between singly excited determinants and more highly excited configurations. Consequently, excitation energies can be obtained to a good approximation by diagonalizing the transformed Hamiltonian in the space of single excitations only. The STEOM method is applied to obtain the valence excitation spectrum of the pyridine molecule. The accuracy of STEOM is shown to be comparable to current state of the art methods like equation-of-motion coupled-cluster theory and CASPT2, whereas the computational requirements of STEOM are very modest compared to the above methods.

Analytic evaluation of energy gradients for the singles and doubles coupled cluster method including perturbative triple excitations: Theory and applications to FOOF and Cr2

Abstract: The analytic energy gradient for the singles and doubles coupled cluster method including a perturbative correction due to triple excitations [CCSD(T)] is formulated and computationally implemented. Encouraged by the recent success in reproducing the experimental equilibrium structure and vibrational frequencies of ozone, the new CCSD(T) gradient method is tested with two other ‘‘difficult’’ quantum chemistry problems: FOOF and Cr2. With the largest basis set employed in this work [triple zeta plus two sets of polarization functions (TZ2Pf)] at the CCSD(T) level of theory, the predictions for the O–O and O–F bond lengths in FOOF are 1.218 and 1.589 A, respectively. These figures are in good agreement with the experimental values 1.216 and 1.575 A. Based on CCSD calculations with even larger basis sets, it is concluded that the error of 0.014 A in the O–F bond length at the TZ2Pf/CCSD(T) level of theory is due to the remaining basis set deficiency. On the other hand, the CCSD(T) prediction for the equilibrium bond length of Cr2 (1.604 A), obtained with a large (10s8p3d2f1g) basis set capable of achieving the Hartree–Fock limit, is still 0.075 A shorter than experiment, clearly indicating the importance of higher than connected triple excitations in a single‐reference treatment of this particular problem.

The current state of ab initio calculations of optical rotation and electronic circular dichroism spectra.

Abstract: The current ability of ab initio models to compute chiroptical properties such as optical rotatory dispersion and electronic circular dichroism spectra is reviewed. Comparison between coupled cluster linear response theory and experimental data (both gas and liquid phase) yields encouraging results for small to medium-sized chiral molecules including rigid species such as (S)-2-chloropropionitrile and (P)-[4]triangulane, as well as conformationally flexible molecules such as (R)-epichlorohydrin. More problematic comparisons are offered by (S)-methyloxirane, (S)-methylthiirane, and (1S,4S)-norbornenone, for which the comparison between theory and experiment is much poorer. The impact of basis-set incompleteness, electron correlation, zero-point vibration, and temperature are discussed. In addition, future prospects and obstacles for the development of efficient and reliable quantum chemical models of optical activity are discussed, including the problem of gauge invariance, scaling of the coupled cluster approach with system size, and solvation.