Towards a pair natural orbital coupled cluster method for excited states (original) (raw)
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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.
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 Chemical Physics, 2021
We present a novel and cost-effective approach of using a second similarity transformation of the Hamiltonian to include the missing higher-order terms in the second-order approximate coupled cluster singles and doubles (CC2) model. The performance of the newly developed ST-EOM-CC2 model has been investigated for the calculation of excitation energies of valence, Rydberg, and chargetransfer excited states. The method shows significant improvement in the excitation energies of Rydberg and charge-transfer excited states as compared to the conventional CC2 method while retaining the good performance of the latter for the valence excited state. The method retains the charge-transfer separability of the CT excited states, which is a significant advantage over the traditional CC2 method. An MBPT2 variant of the new method is also proposed.
International Journal of Quantum Chemistry, 1998
The impact of the choice of molecular orbital sets on the results of Ž . single-reference-state coupled-cluster CC methods was studied for the H4 model. This model offers a straightforward way of taking into account all possible symmetry-adapted orbitals. Moreover, the degree of quasi-degeneracy of its ground state can be varied over a wide range by changing its geometry. The CCD, CCSD, and CCSDT approaches are considered. Surfaces representing the dependence of the energy on the parameters defining the orbitals are obtained. It is documented that for every method there exist alternative orbital sets which allow one to obtain more accurate energies than the Ž . standard HF, BO, and NO ones. However, for many of the former orbital sets, one obtains relatively large one-body amplitudes or one may encounter problems with solving the CC equations by conventional methods. An interesting variety of orbitals which might be useful for studies of quasi-degenerate states by the CCD method was found.
Journal of Chemical Theory and Computation, 2007
Several variants of the equation-of-motion coupled-cluster (EOM-CC) method with singles (one-hole or one-particle), doubles (two-hole-one-particle or two-particle-one-hole), and a selected set of triples (three-hole-two-particle or three-particle-two-hole) and/or quadruples (four-hole-three-particle or four-particle-three-hole) have been implemented by computerized symbolic algebra. They are applicable to excitation energies (EE), ionization potentials (IP), and electron affinities (EA), excited-state dipole moments, and transition dipole moments of both closed-and open-shell species and are abbreviated as EE/IP/EA-EOM-CCSDt, EE/IP/ EA-EOM-CCSDtq, and EE/IP/EA-EOM-CCSDTq, where the small letters indicate the use of active-space cluster and EE/IP/EA operators. They are also parallel executable and accelerated by the use of spin, spatial, and permutation symmetries. The remarkable effectiveness of the methods in capturing nondynamical correlation effects has been demonstrated by their applications to the vertical excitation energies of C 2 , the adiabatic excitation energies and dipole moments of the CH radical, the adiabatic excitation energies of the CH 2 diradical, the adiabatic excitation energies and dipole moments of formaldehyde, the vertical ionization energies of N 2 , and the vertical electron affinities of C 2. The effectiveness is found to decline when the basis set is extended, causing the active space to become relatively small and also less well-defined. As a remedy, we propose a composite method that combines higher-rank active-space methods with smaller basis sets for nondynamical correlation and lower-rank nonactive-space methods with larger basis sets for dynamical correlation, which is shown to work well for an excited-state potential energy curve of hydrogen fluoride.
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 B.V.
A local similarity transformed equation of motion approach for calculating excited states
International Journal of Quantum Chemistry, 2020
The efficient and accurate calculation of excitation energies and properties for large molecular systems remains a challenge. In this perspective, local implementation of the similarity-transformed equation of motion-coupled cluster method will be briefly outlined, and its current uses and future potentials will be shortly summarized. The available calculations using this new method suggest that it can be applied to a variety of large systems, for which it delivers accurate results.
A near-linear scaling equation of motion coupled cluster method for ionized states
The Journal of chemical physics, 2018
In this work, a domain-based local pair natural orbital (DLPNO) version of the equation of motion coupled cluster theory with single and double excitations for ionization potentials (IP-EOM-CCSD) equations has been formulated and implemented. The method uses ground state localized occupied and pair natural virtual orbitals and applies the DLPNO machinery to arrive at a linear scaling implementation of the IP-EOM-CCSD method. The accuracy of the method is controllable using ground state truncation parameters. Using default thresholds, the method predicts ionization potential (IP) values with good accuracy (mean absolute error of 0.08 eV). We demonstrate that our code can be used to compute IP values for systems with more than 1000 atoms and 10 000 basis functions.