Coupled cluster method equations

The use of Cl methods has been declining in recent years, to the profit of MP and especially CC methods. It is now recognized that size extensivity is important for obtaining accurate results. Excited states, however, are somewhat difficult to treat by perturbation or coupled cluster methods, and Cl or MCSCF based methods have been the prefen ed methods here. More recently propagator or equation of motion (Section 10.9) methods have been developed for coupled cluster wave functions, which allows calculation of exited state properties. 

Stanton JF, Bartlett RJ (1993) The equation of motion coupled-cluster method - a systematic biorthogonal approach to molecular-excitation energies, transition-probabilities, and excited-state properties. J Chem Phys 98 7029... 

The relativistic coupled cluster method starts from the four-component solutions of the Drrac-Fock or Dirac-Fock-Breit equations, and correlates them by the coupled-cluster approach. The Fock-space coupled-cluster method yields atomic transition energies in good agreement (usually better than 0.1 eV) with known experimental values. This is demonstrated here by the electron affinities of group-13 atoms. Properties of superheavy atoms which are not known experimentally can be predicted. Here we show that the rare gas eka-radon (element 118) will have a positive electron affinity. One-, two-, and four-components methods are described and applied to several states of CdH and its ions. Methods for calculating properties other than energy are discussed, and the electric field gradients of Cl, Br, and I, required to extract nuclear quadrupoles from experimental data, are calculated. 

Shiozaki, T., Kamiya, M., Hirata, S., Valeev, E.F. Equations of explicitly-correlated coupled-cluster methods. Phys. Chem. Chem. Phys. 2008, 10, 3358-70. 

Hirata, S. Higher-order equation-of-motion coupled-cluster methods. J. Chem. Phys. 2004, 121, 51-9. 

BASIC ELEMENTS OF COUPLED-CLUSTER AND EQUATION-OF-MOTION COUPLED-CLUSTER METHODS... 

Inclusion of connected triple excitations in the equation-of-motion coupled-cluster method for calculating excitation energies was also tested on SiFP230. Excitation energies predicted for the lowest triplet state and the two lowest excited singlet states 1 Si and 2 lAi were within a kcal mol 1 of the predictions of the full Cl calculation when triple excitations were included, but only for the 2 3Ai state was there marked improvement over the inclusion of only single and double excitations. 

J. Geersten, M. Rittby, R.J. Bartlett, The equation-of-motion coupled-cluster method Excitation energies of Be and CO, Chem. Phys. Lett. 164 (1989) 57 D.C. Comeau, R.J. Bartlett, The equation-of-motion coupled-cluster method. Applications to open- and closed-shell reference states, Chem. Phys. Lett. 207 (1993) 414. 

More demanding calculations conducted by Daniel et al. using equations-of-motion coupled cluster methods (EOM-CC) provided a better description of the contribution of MLCT andMC transitions to the electronic spectrum of Cr(CO)6 [13]. These calculations confirm that the lowest-energy transition, which is responsible for the shoulder on the low-energy side of the first absorption maximum, corresponds to... 

AN INTRODUCTION TO EQUATION-OF-MOTION AND LINEAR-RESPONSE COUPLED-CLUSTER METHODS FOR ELECTRONICALLY EXCITED STATES OF MOLECULES... 

Watts JD, Bartlett RJ (1995) Economical triple excitation equation-of-motion coupled-cluster methods for excitation energies. Chem Phys Lett 233 81-87. 

Kowalski K, Piecuch P (2000) The active-space equation-of-motion coupled-cluster methods for excited states The EOMCCSDt approach. J Chem Phys 113 8490-8502. 

Stanton JF (1993) Many-body methods for excited state potential energy surfaces I. General theory of energy gradients for the equation-of-motion coupled-cluster method. J Chem Phys 99 8840-8847. 

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