A simple scheme for calculating approximate transition moments within the equation of motion expectation value formalism

Abstract

A simple scheme for calculating approximate transition moments within the framework of the equation of motion coupled cluster method is proposed. It relies on a matrix inversion technique to calculate the excited state left eigenvectors and requires no additional cost over that of the excitation energy calculation. The new approximation gives almost identical UV-Vis spectra to that obtained using the standard equation of motion coupled cluster method with single and double excitations for molecules in a standard test set.

Bound anionic states of DNA and RNA nucleobases: An EOM‐CCSD investigation

“…The presence of additional diffuse functions can lead to numerical instabilities in the COSX‐based evaluation of the four external integral and results in convergence issue in the EOM‐CCSD iterations. The default radial grid of 2.68 is not accurate enough to evaluate integrals over the diffuse basis functions, which decays very slowly with distance from the atom centers. One can bypass this problem by increasing the radial grid size in the COSX integration, and a radial grid of 25 has been used for this study.…”

“…The initial attempts were all based on the perturbative truncation of the similarity‐transformed Hamiltonian and EOM matrix. However, the accuracy of these perturbative approximations is often not consistent and can give errors as high as 0.5 eV, even for the molecules where the Hartree‐Fock method gives a correct zero‐order description of the reference wave function.…”

“…It makes the wave function for different excited states biorthonormal to one another and evaluation of properties becomes simpler than the state‐specific approaches. A further gain in computation efficiency is obtained using the chain of sphere exchange (COSX) approximation for the evaluation of the four external terms, calculation of which is the most expensive step in the bt ‐PNO‐EOM‐CCSD method. The use of COSX approximation reduces formal scaling of the term involving four external integrals and removes the associated storage bottlenecks.…”

“…The use of TIGHTPNO setting gives results within 14 meV of canonical EOM‐CCSD for the EA values, and all the calculations in this study have been performed using the bt ‐PNO‐EA‐EOM‐CCSD method with TIGHTPNO setting. The numerical errors in the calculated EA also dependent on the grid settings in seminumerical COSX approximation, and default grid settings of the bt ‐PNO‐EA‐EOM‐CCSD generally lead to an error bar of less than 1 meV with respect to the canonical EA‐EOM‐CCSD values . However, some electron attached states can be more sensitive to grid settings, especially in the presence of diffuse functions.…”

Segregation of tetracarbon units in low‐energy tetracarbindane structures: Major differences from their aluminum and gallium analogs

“…The energetically most stable isomers were further optimized by employing the PBE0 DFT functional and the def2‐TZVP basis set. In order to refine further the energies, coupled‐cluster single‐point energy calculations were performed on the lowest energy isomers using the domain‐based local pair‐natural orbital coupled‐cluster method including single and double excitations and perturbative correction for connected triples (DLPNO‐CCSD[T]) coupled with the def2‐QZVP basis set as implemented in the ORCA 3.0.3 software package . All of the resulting optimized structures were found to have substantial HOMO‐LUMO gaps (cf.…”