Towards a pair natural orbital coupled cluster method for excited states

Abstract

The use of back-transformed pair natural orbitals in the calculation of excited state energies, ionization potentials, and electron affinities is investigated within the framework of equation of motion coupled cluster theory and its similarity transformed variant. Possible approaches to a more optimal use of pair natural orbitals in these methods are indicated.

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

“…However, the recent back‐transformed natural orbital approach of Dutta et al can overcome these shortcomings and reduce the computational cost and storage requirement of the coupled cluster method without significant compromise in its accuracy.…”

“…Peng and Valeev have recently reported an efficient EOM‐CCSD implementation based on state‐averaged PNOs . As an alternative strategy, Neese and coworkers have persuaded back‐transformed PNO approach ( bt ‐PNO). In this approach after the solution of the ground state DLPNO‐CCSD equations, amplitudes are back‐transformed to canonical basis, and the EOM part is calculated in the canonical basis.…”

“…The use of COSX approximation reduces formal scaling of the term involving four external integrals and removes the associated storage bottlenecks. The readers are requested to consult reference for more details of the bt ‐PNO‐EOM‐CCSD method.…”

“…These three keywords give progressively more accurate energy (or energy differences) at the expense of higher computational cost. 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 .…”

“…The study of bound anionic state requires a large and flexible basis set, which is difficult to use in standard EOM‐CCSD calculations owing to their prohibitively high cost. However, the recent developments in local correlation‐based EOM‐CCSD method have made it possible to calculate electron affinities of large molecules with systematically controllable accuracy. The aim of this article is to study the bound anionic states of NAB using the local correlation‐based EOM‐CCSD method and flexible basis sets.…”