Universität zu Köln

Herrmann, Nils ORCID: 0000-0002-6468-9180 and Hanrath, Michael (2022). A correctly scaling rigorously spin-adapted and spin-complete open-shell CCSD implementation for arbitrary high-spin states. J. Chem. Phys., 156 (5). MELVILLE: AIP Publishing. ISSN 1089-7690

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Abstract

In this paper, we report on a correctly scaling novel coupled cluster singles and doubles (CCSD) implementation for arbitrary high-spin open-shell states. The chosen cluster operator is completely spin-free, i.e., employs spatial substitutions only. It is composed of our recently developed Lowdin-type operators [N. Herrmann and M. Hanrath, J. Chem. Phys. 153, 164114 (2020)], which ensure (1) spin completeness and (2) spin adaption, i.e., spin purity of the CC wave function. In contrast to the proof-of-concept matrix-representation-based implementation presented there, the present implementation relies on second quantization and factorized tensor contractions. The generated singles and doubles operators are embedded in an equation generation engine. In the latter, Wick's theorem is used to derive prefactors arising from spin integration directly from the spin-free full contraction patterns. The obtained Wick terms composed of products of Kronecker deltas are represented by special non-antisymmetrized Goldstone diagrams. Identical (redundant) diagrams are identified by solving the underlying graph isomorphism problem. All non-redundant graphs are then automatically translated to locally-one term at a time-factorized tensor contractions. Finally, the spin-adapted and spin-complete (SASC) CCS and CCSD variants are applied to a set of small molecular test systems. Both correlation energies and amplitude norms hint toward a reasonable convergence of the SASC-CCSD method for a Baker-Campbell-Hausdorff series truncation of order four. In comparison to spin orbital CCSD, SASC-CCSD leads to slightly improved correlation energies with differences of up to 1.292mE(H) (1.10% with respect to full configuration identification) for quintet CH2 in the cc-pVDZ basis set.

Item Type:	Journal Article
Creators:	Creators Email ORCID ORCID Put Code Herrmann, Nils UNSPECIFIED orcid.org/0000-0002-6468-9180 UNSPECIFIED Hanrath, Michael UNSPECIFIED UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-658447
DOI:	10.1063/5.0078020
Journal or Publication Title:	J. Chem. Phys.
Volume:	156
Number:	5
Date:	2022
Publisher:	AIP Publishing
Place of Publication:	MELVILLE
ISSN:	1089-7690
Language:	English
Faculty:	Unspecified
Divisions:	Unspecified
Subjects:	no entry
Uncontrolled Keywords:	Keywords Language COUPLED-CLUSTER THEORY; LINEAR-RESPONSE CALCULATION; WAVE-FUNCTION; AUTOMATED IMPLEMENTATION; APPROXIMATE ACCOUNT; PERTURBATION-THEORY; MOLECULAR-SYSTEMS; EXCITED CLUSTERS; SINGLE; CONTAMINATION Multiple languages Chemistry, Physical; Physics, Atomic, Molecular & Chemical Multiple languages
URI:	http://kups.ub.uni-koeln.de/id/eprint/65844