Abstract
The recently developed Local Framework for calculating Excitation energies (LoFEx) is extended to the coupled cluster singles and doubles (CCSD) model. In the new scheme, a standard CCSD excitation energy calculation is carried out within a reduced excitation orbital space (XOS), which is composed of localised molecular orbitals and natural transition orbitals determined from time-dependent Hartree–Fock theory. The presented algorithm uses a series of reduced second-order approximate coupled cluster singles and doubles (CC2) calculations to optimise the XOS in a black-box manner. This ensures that the requested CCSD excitation energies have been determined to a predefined accuracy compared to a conventional CCSD calculation. We present numerical LoFEx-CCSD results for a set of medium-sized organic molecules, which illustrate the black-box nature of the approach and the computational savings obtained for transitions that are local compared to the size of the molecule. In fact, for such local transitions, the LoFEx-CCSD scheme can be applied to molecular systems where a conventional CCSD implementation is intractable.
| Original language | English |
|---|---|
| Journal | Molecular Physics |
| Volume | 115 |
| Issue | 17-18 |
| Pages (from-to) | 2135-2144 |
| Number of pages | 10 |
| ISSN | 0026-8976 |
| DOIs | |
| Publication status | Published - 17 Sept 2017 |
Keywords
- CCSD
- Excitation energies
- coupled cluster theory
- large molecules
- local correlation