Open-shell coupled-cluster method: Direct calculation of excitation energies (original) (raw)
WIREs Computational Molecular Science, 2019
While methodological developments in the last decade made it possible to compute coupled cluster (CC) energies including excitations up to a perturbative triples correction for molecules containing several hundred atoms, a similar breakthrough has not yet been reported for excited state computations. Accurate CC methods for excited states are still expensive, although some promising candidates for an efficient and accurate excited state CC method have emerged recently. This review examines the various approximation schemes with particular emphasis on their performance for excitation energies and summarizes the best state-of-the-art results which may pave the way for a robust excited state method applicable to molecules of hundreds of atoms. Among these, special attention will be given to exploiting the techniques of similarity transformation, perturbative approximations as well as integral decomposition, local and embedding techniques within the equation of motion CC framework.
Chemical Physics Letters, 1986
For excitation energy calculations using quasidegenerate MBPT or coupled-cluster (CC) theory, the hitherto chosen strategy has been to have particle-hole (p-h) determinants forming the model space and to use standard formalisms origlnally developed for complete model spaces. In view of our recent analysis, the p-h determinants constitute an incomplete model space for which intermediate normalization (IN) of the wave-operator n is not appropriate if a linked formulation is deshed. The theoretical status of earlier applications which had used formulae appropriate to IN, yet ignored disconnected diagrams, is analyzed. It is shown that if only excitation energies, rather than the full 52, are desired, then no theoretical error is made. The situation is, however, fortuitous in that for mp-mh model spaces with m > 1, a similar procedure using IN would necessarily generate disconnected diagrams.
Direct evaluation of one-electron properties in coupled cluster methods
Theoretica Chimica Acta, 1990
An analysis of a method for approximate calculations of expectation values for one-electron operators from available coupled cluster amplitudes is presented and illustrated numerically for the polarizability of the Be atom. The one-particle density matrix resulting from the present approach is accurate through the fourth order in the electron correlation perturbation. It has been found that, in order to obtain quantitative agreement between the energy derivative results and the approximate expectation value formalism, the third order TIT21,(°)> wave function term must be included into the calculation of the one-particle density matrix. The present method is also considered as a promising tool for calculations of higher-order atomic and molecular properties from high level correlated wave functions.
Partition of electronic excitation energies: the IQA/EOM-CCSD method
Physical Chemistry Chemical Physics, 2019
We put together equation of motion coupled cluster theory and the interacting quantum atoms electronic energy partition to determine how an absorbed photon changes atomic energies as well as covalent and noncovalent interactions within a molecule or molecular cluster.
Simplified methods for equation-of-motion coupled-cluster excited state calculations
Chemical Physics Letters, 1996
Simplified equation-of-motion coupled-cluster (EOM-CC) methods derived from matrix partitioning and perturbation approximations are presented and applied to a variety of molecules. By combining a partitioned EOM-CC method with an MBPT(2) treatment of the ground state, we obtain an iterative n s method which gives excitation energies that normally fall within 0.2 eV of the full EOM-CCSD excitation energy. Results are shown to be superior to other simplified approaches that have been proposed.
Http Dx Doi Org 10 1080 00268976 2013 858192, 2014
Using an asymmetric Lanczos chain algorithm for the calculation of the coupled cluster linear response functions at the coupled cluster singles and doubles (CCSD) and coupled cluster singles and approximate iterative doubles (CC2) levels of approximation, we have calculated the mean excitation energies of the noble gases He, Ne and Ar, and of the hydrogen molecule (H2). Convergence with respect to the one-electron basis set was investigated in detail for families of correlation-consistent basis sets including both augmentation and core-valence functions. We find that the electron correlation effects at the CCSD level change the mean excitation energies obtained at the uncorrelated Hartree–Fock level by about 1%. For the two-electron systems He and H2, our CCSD results (for a Lanczos chain length equal to the full excitation space), I0 = 42.28 eV (helium) and I0 = 19.62 eV (H2), correspond to full configuration interaction results and are therefore the exact, non-relativistic theoretical values for the mean excitation energy of these two systems within the Bethe theory for the chosen basis set and, in the case of H2, at the experimental equilibrium geometry.
Chemical Physics Letters, 2000
General-order equation-of-motion coupled-cluster methods for ionization potentials and electron anities (IP-EOM-CC and EA-EOM-CC) are developed by employing a determinantal algorithm. With these, principal ionization potentials or electron anities of diatomic molecules and the excitation energies of their ionized or electron-attached counterparts are computed across dierent approximations of the cluster operator and the ionization (electron-attachment) operator. IP-EOM-CC(2,2h-1p) IP-EOM-CCSD and EA-EOM-CC(2,1h-2p) EA-EOM-CCSD or EA-EOM-CC(2,2h-3p) prove to be well-balanced models for principal ionization potentials and electron anities, whereas for the quantitative descriptions of non-Koopmans ionization or electron-attachment processes IP-EOM-CC(3, 3h-2p) IP-EOM-CCSDT and EA-EOM-CC(2,2h-3p) appear to be the minimal levels. Ó 2000 Elsevier Science (R.J. Bartlett). 0009-2614/00/$ -see front matter Ó 2000 Elsevier Science B.V. PII: S 0 0 0 9 -2 6 1 4 ( 0 0 ) 0 0 9 6 5 -9
The Journal of Physical Chemistry, 1988
In order to circumvent the problem of spin contamination in unrestricted Hartree-Fock based coupled cluster (CC) calculations, we present a new method of calculation for certain classes of open-shell systems. The approach ensures that the proper spin component of the resulting correlated wave function is projected out in the energy evaluation by the use of a reference function constructed from suitably chosen restricted open-shell Hartree-Fock or other orbitals. This single-reference open-shell spin-restricted CC method is applied to the calculation of ionization potentials in the N2 molecule, and it is shown that highly accurate results can be obtained in a 5s4pld basis. The mean error for all the principal ionization potentials of N2 compared to experiment is 0.45%.