Coupled cluster

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

Analytic gradients for excited states in the coupled-cluster model CC2 employing the resolution-of-the-identity approximation

Abstract: The derivation and implementation of excited state gradients is reported for the approximate coupled-cluster singles and doubles model CC2 employing the resolution-of-the-identity approximation for electron repulsion integrals. The implementation is profiled for a set of examples with up to 1348 basis functions and exhibits no I/O bottlenecks. A test set of sample molecules is used to assess the performance of the CC2 model for adiabatic excitation energies, excited state structure constants and vibrational frequencies. We find very promising results, especially for adiabatic excitation energies, though the need of a single-reference ground state and a single-replacement dominated excited state puts some limits on the applicability of the method. Its reliability, however, can always be tested on grounds of diagnostic measures. As an example application, we present calculations on the π*←π excited state of trans-azobenzene.

Ab initio calculation of molecular chiroptical properties

Abstract: This review describes the first-principles calculation of chiroptical properties such as optical rotation, electronic and vibrational circular dichroism, and Raman optical activity. Recent years have witnessed a flurry of activity in this area, especially in the advancement of density-functional and coupled cluster methods, with two ultimate goals: the elucidation of the fundamental relationship between chiroptical properties and detailed molecular structure, and the development of a suite of computational tools for the assignment of the absolute configurations of chiral molecules. The underlying theory and the basic principles of such calculations are given for each property, and a number of representative applications are discussed.

The electronic couplings in electron transfer and excitation energy transfer.

Abstract: The transport of charge via electrons and the transport of excitation energy via excitons are two processes of fundamental importance in diverse areas of research. Characterization of electron transfer (ET) and excitation energy transfer (EET) rates are essential for a full understanding of, for instance, biological systems (such as respiration and photosynthesis) and opto-electronic devices (which interconvert electric and light energy). In this Account, we examine one of the parameters, the electronic coupling factor, for which reliable values are critical in determining transfer rates. Although ET and EET are different processes, many strategies for calculating the couplings share common themes. We emphasize the similarities in basic assumptions between the computational methods for the ET and EET couplings, examine the differences, and summarize the properties, advantages, and limits of the different computational methods. The electronic coupling factor is an off-diagonal Hamiltonian matrix element between the initial and final diabatic states in the transport processes. ET coupling is essentially the interaction of the two molecular orbitals (MOs) where the electron occupancy is changed. Singlet excitation energy transfer (SEET), however, contains a Frster dipole-dipole coupling as its most important constituent. Triplet excitation energy transfer (TEET) involves an exchange of two electrons of different spin and energy; thus, it is like an overlap interaction of two pairs of MOs. Strategies for calculating ET and EET couplings can be classified as (1) energy-gap-based approaches, (2) direct calculation of the off-diagonal matrix elements, or (3) use of an additional operator to describe the extent of charge or excitation localization and to calculate the coupling value. Some of the difficulties in calculating the couplings were recently resolved. Methods were developed to remove the nondynamical correlation problem from the highly precise coupled cluster models for ET coupling. It is now possible to obtain reliable ET couplings from entry-level excited-state Hamiltonians. A scheme to calculate the EET coupling in a general class of systems, regardless of the contributing terms, was also developed. In the past, empirically derived parameters were heavily invoked in model description of charge and excitation energy drifts in a solid-state device. Recent advances, including the methods described in this Account, permit the first-principle quantum mechanical characterization of one class of the parameters in such descriptions, enhancing the predictive power and allowing a deeper understanding of the systems involved.

Recent Progress in MANY-BODY THEORIES

Abstract: Ludwig Boltzmann: Recent Applications of Boltzmann's Theory (D. Rainer). Cycles of the Transition Processes as Basic Idea of Ludwig Boltzmann (V.I. Belinicher). Quantum Fluids: Recent Progress in the Theory of Highly Correlated Quantum Fluids (C.E. Campbell). Systematics and Numerics in Manybody Systems (M. Suzuki). Quantum Chemistry: Research of Appropriate Treatments of the Various Aspects of the Electron Correlation in Molecules and Their Interplay (J.P. Malrieu). A Coupled Cluster Approach to the Electron Correlation Problem Using a Correlated Reference State (D. Mukherjee). Nuclear Physics: Realistic Microscopic Calculations of Nuclear Structure (B.R. Barrett et al.). Microscopic Theories of Atomic and Nuclear Optical Potentials (C. Mahaux). Lattice Hamiltonians: Microscopic Theories of Quantum Lattice Systems (R.F. Bishop). A Nonperturbative Microscopic Theory of Hamiltonian Lattice Gauge Systems (R.F. Bishop et al.). Complex Systems: Complexity, Correlations, and Fluctuations in Manyparticle Systems (P.A. Carruthers). Solids: High Temperature Superconductors: A Review (E. Dagotto). Feenberg Medal Presentation: Pathways to the Quantum Realm (J.W. Clark). 28 additional articles. Index.