Coupled-cluster method with singles and doubles

Approximate Coupled Cluster Methods Combined Reduced Multireference and Almost-Linear Coupled Cluster Methods with Singles and Doubles ... 

K. B. Bravaya, D. Zuev, E. Epifanovsky, and A. I. Krylov,/. Chem. Phys., 138,124106 1-15 (2013). Complex-Scaled Equation-of-Motion Coupled-Cluster Method with Single and Double Substitutions for Autoionizing Excited States Theory, Implementation, and Examples. 

The QCISD method is also very closely related to coupled cluster theory, with singles and doubles (CCSD). In contrast to QCISD. 

Instead, practical methods involve a subset of possible Slater determinants, especially those in which two electrons are moved from the orbitals they occupy in the HF wavefunction into empty orbitals. These doubly excited determinants provide a description of the physical effect missing in HF theory, correlation between the motions of different electrons. Single and triple excitations are also included in some correlated ab initio methods. Different methods use different techniques to decide which determinants to include, and all these methods are computationally more expensive than HF theory, in some cases considerably more. Single-reference correlated methods start from the HF wavefunction and include various excited determinants. Important methods in inorganic chemistry include Mpller-Plesset perturbation theory (MP2), coupled cluster theory with single and double excitations (CCSD), and a modified form of CCSD that also accounts approximately for triple excitations, CCSD(T). 

A superior method for the calculation of excited-state PE surfaces is CC2, which is a simplified and computationally efficient variant of coupled-cluster theory with single and double excitations [22], CC2 can be considered as the equivalent of MP2 for excited electronic states. Efficient implementations of CC2 with density fitting [23] and analytic gradients [24] allow reaction path calculations for rather large systems. Being a singlereference method, CC2 fails in the vicinity of conical intersections of excited states with the electronic ground state. 

The accuracy of the CCSD(T) method for strongly bound molecules is illustrated in Figure 5. This figure provides a statistical analysis of the errors in the computed values for a representative group of molecules. The curves represent the normal error distributions for three different methods commonly used to solve the electronic Schrodinger equation second-order Mpller-Plesset perturbation theory (MP2), coupled cluster theory with single and double excitations, and... 

For Three Molecules in Valence Double-Zeta Basis Sets, a Comparison of Energies in Hartrees (H) from the 2-RDM Method with the T2 Condition (DQGT2) with the Energies from Second-Order Many-Body Perturbation Theory (MP2), Coupled-Cluster Method with Single-Double Excitations and a Perturbative Triples Correction (CCSD(T)), and Full Configuration Interaction (FCI)... 

Two approaches to electron correlation that are widely used today for the studies of organic radical cations are Coupled Cluster (CC) calculations or the similar, but not identical, Quadratic Configuration Interaction (QCI) method with single and double excitations, often followed by CCSD(T) or QCISD(T) single point calculations with a larger basis set. These methods suffer to a much lesser extent from... 

Kowalksi K, Piecuch P (2004) New coupled-cluster methods with singles, doubles, and noniterative triples for high accuracy calculations of excited electronic states. J Chem Phys 120 1715-1738. 

Demel, O. Pittner, J. Multireference Brillouin-Wigner coupled cluster method with singles, doubles, and triples efficient implementation and comparison with approximate approaches, 7. Chem. Phys. 2008,128,104108-104111. 

A different study provides some further information coneeming the effects of electron correlation upon calculated intensities. In addition to SCF and MPn type methods, these authors considered also the coupled-cluster model which is an infinite-order generalization of MR Coupled-cluster, limited to single and double excitations (CCSD), was considered, as well as CCSD + T which includes the effects of connected triple excitations. The doubleharmonic approximation was made, consistent with most calculations of H-bonded systems. The [5s3pld/3slp] basis set is of the polarized double- variety, within reach of most workers. 

The CC2 method [74] is an approximation to coupled cluster with singles and doubles (CCSD), and the excited state energies calculated have MP2 quality. An implementation that employs the resolution of identity (RI) approximation for two-electron integrals to reduce the CPU time is also available, RI-CC2 [75], which is suitable for large scale integral-direct calculations. This method has been implemented in TURBOMOLE [76],... 

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