Multilevel CC2 and CCSD in Reduced Orbital Spaces: Electronic Excitations in Large Molecular Systems

Abstract

We present efficient implementations of the multilevel CC2 (MLCC2) and multilevel CCSD (MLCCSD) models. As the system size increases, MLCC2 and MLCCSD exhibit the scaling of the lower-level coupled cluster model. To treat large systems, we combine MLCC2 and MLCCSD with a reduced-space approach in which the multilevel coupled cluster calculation is performed in a significantly truncated molecular orbital basis. The truncation scheme is based on the selection of an active region of the molecular system and the subsequent construction of localized Hartree–Fock orbitals. These orbitals are used in the multilevel coupled cluster calculation. The electron repulsion integrals are Cholesky decomposed using a screening protocol that guarantees accuracy in the truncated molecular orbital basis and reduces computational cost. The Cholesky factors are constructed directly in the truncated basis, ensuring low storage requirements. Systems for which Hartree–Fock is too expensive can be treated by using a multilevel Hartree–Fock reference. With the reduced-space approach, we can handle systems with more than a thousand atoms. This is demonstrated for paranitroaniline in aqueous solution.

Excited state absorption of DNA bases in the gas phase and in chloroform solution: a comparative quantum mechanical study

“…Yet, very encouraging results on the treatment of larger systems, including solvated systems, with CC accuracy come from the latest advances in multilevel coupled cluster theory. 62–65…”

Transient Absorption of DNA bases in the gas phase and in chloroform solution: a comparative quantum mechanical study

“…Yet, very encouraging results on the treatment of larger systems, including solvated systems, with CC accuracy come from the latest advances in multilevel coupled cluster theory. [60][61][62][63] Another interesting feature of TD-CAM-B3LYP is the relatively small dependence of the computed spectra on the size of the basis set. As shown in the SI, the spectra obtained at the TD-CAM-B3LYP/6-31G(d) level are fairly similar to the ones reported here, but for a moderate, almost uniform, blue-shift.…”

Oscillator strengths in the framework of equation of motion multilevel CC3

“…Projected atomic orbitals have been shown to give a good description of the virtual space for solvated systems, but also adenosine. 66,74,75 The construction of correlated natural transition orbitals is more costly as they are obtained from excitation vectors of a coupled cluster calculation. In MLCC3 we use CNTOs constructed from CCSD excited states to get a compact description of the excited states.…”