Coupled-cluster theory configuration

Keywords Many-body theory Brillouin-Wigner theory State-specific multireference correlation problem Many-body perturbation theory Coupled cluster theory Configuration interaction Collaborative virtual enviroments... 

In practice, the different expansions for the correlation energy afforded by coupled cluster theory, configuration interaction and perturbation theory have to be truncated in order to render computations tractable. The different methods differ only in the way in which this truncation is carried out. However,... 

Various ab initio methods in GAUSSIAN can generate. wfx files for DDEC analysis. These ab initio calculations can be performed in a vacuum or in an implicit solvent. DDEC analysis should work with any of these ab initio methods. We strongly recommend using methods such as DFT, coupled-cluster theory, configuration interaction, etc. that accurately account for electron electron correlations. Low-level theories that do not accurately account for... 

There is also a hierarchy of electron correlation procedures. The Hartree-Fock (HF) approximation neglects correlation of electrons with antiparallel spins. Increasing levels of accuracy of electron correlation treatment are achieved by Mpller-Plesset perturbation theory truncated at the second (MP2), third (MP3), or fourth (MP4) order. Further inclusion of electron correlation is achieved by methods such as quadratic configuration interaction with single, double, and (perturbatively calculated) triple excitations [QCISD(T)], and by the analogous coupled cluster theory [CCSD(T)] [8],... 

Coupled-Cluster Theory with Singles, Doubles and Noniterative Approximation of Triples Configuration Interaction... 

The exact FCI (frill configuration interaction) solution of the PPP or Hubbard model is possible for molecules with up to about 16 atoms in the pi system. Any of the standard methods for performing approximate ab initio calculations, such as limited configuration interaction, Moeller-Plesset perturbation theory, or coupled cluster theory, may be applied to these models as well. All are expected to be very accurate at low order when U is small, but all will have to be pushed to higher order as U increases. 

Rico RJ, Lee TJ, Head-Gordon M (1994) The origin of differences between coupled cluster theory and quadratic configuration interaction for excited states. Chem Phys Lett 218 139-146. 

Larsen H, Hald K, Olsen J, J0rgensen P (2001) Triplet excitation energies in full configuration interaction and coupled-cluster theory. J Chem Phys 115 3015-3020. 

Christiansen O, Koch H, Jprgensen P, Olsen J (1996) Excitation energies of H20, N2, and C2 in full configuration and coupled cluster theory. Chem Phys Lett 256 185-194. 

Optical rotation (OR), optical rotatory dispersion (ORD), electronic circular dichroism (BCD), and vibrational circular dichroism (VCD) provide spectral information nniqne to enantiomers, allowing for the determination of absolute configuration. Recent theoretical developments in DFT, using time-dependent density fnnctional theory (TD-DFT), provide the means for computing OR, ORD, and Similar theoretical development with coupled-cluster theory,... 

With any type of molecular modeling, there is generally a tradeoff between cost and reliability, and one typically shuns models that cost more without increasing reliability. In practice, this cost is usually expressed as computational effort, or computer time. In gas phase modeling, one typically finds molecular mechanics and semiempirical molecular orbital theory at the low-cost end and multireference configuration interaction or coupled-cluster theory at the other, with the choice dictated by the size of the system. System size also influences the choice of solvation model. We consider first the least expensive models, those that take no account of the quantum mechanical nature of the solute. 

In this section we examine some of the critical ideas that contribute to most wavefunction-based models of electron correlation, including coupled cluster, configuration interaction, and many-body perturbation theory. We begin with the concept of the cluster function which may be used to include the effects of electron correlation in the wavefunction. Using a formalism in which the cluster functions are constructed by cluster operators acting on a reference determinant, we justify the use of the exponential ansatz of coupled cluster theory. ... 

In many-electron theories such as configuration interaction or coupled cluster theory, it is more convenient to deal with the -electron reference determinant, IOq), rather than the true vacuum state, I ). In the evaluation of matrix elements using Wick s theorem as described above, even the use of normal-ordered strings would be tremendously tedious if one had to include the complete set of operators required to generate ld>o) from the true vacuum (i.e., lOo) = aUjat I )). 

The set of atomic orbitals Xk is called a basis set, and the quality of the basis set will usually dictate the accuracy of the calculations. For example, the interaction energy between an active site and an adsorbate molecule might be seriously overestimated because of excessive basis set superposition error (BSSE) if the number of atomic orbitals taken in Eq. [4] is too small. Note that Hartree-Fock theory does not describe correlated electron motion. Models that go beyond the FiF approximation and take electron correlation into account are termed post-Flartree-Fock models. Extensive reviews of post-HF models based on configurational interaction (Cl) theory, Moller-Plesset (MP) perturbation theory, and coupled-cluster theory can be found in other chapters of this series. ... 

By ab initio we refer to quantum chemical methods in which all the integrals of the theory, be it variational or perturbative, are exactly evaluated. The level of theory then refers to the type of theory employed. Common levels of theory would include Hartree-Fock, or molecular orbital theory, configuration interaction (Cl) theory, perturbation theory (PT), coupled-cluster theory (CC, or coupled-perturbed many-electron theory, CPMET), etc. - We will use the word model to designate approximations to the Hamiltonian. For example, the zero differential overlap models can be applied at any level of theory. The distinction between semiempirical and ab initio quantum chemistry is often not clean. Basis sets, for example, are empirical in nature, as are effective core potentials. The search for basis set parameters is not usually considered to render a model empirical, whereas the search for parameters in effective core potentials is so considered. 

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