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Energetics of 1, n ‐didehydro‐polyene diradicals and performance of reduced multireference coupled‐cluster method
Author(s) -
Li Xiangzhu,
Paldus Josef
Publication year - 2009
Publication title -
international journal of quantum chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.484
H-Index - 105
eISSN - 1097-461X
pISSN - 0020-7608
DOI - 10.1002/qua.22186
Subject(s) - diradical , coupled cluster , chemistry , wave function , singlet state , multireference configuration interaction , triplet state , polyene , atomic physics , excited state , computational chemistry , potential energy , ground state , configuration interaction , physics , molecule , organic chemistry
The reduced multireference coupled‐cluster method with singles and doubles (RMR CCSD) and its RMR CCSD(T) version are employed to study the energetics of 1, n ‐didehydro‐polyenes. The RMR CCSD method accounts for quasi‐degeneracy by exploiting a multireference (MR) CISD wave function as an external source of the most important (primary) triples and quadruples while the subsequent perturbative correction for the secondary triples leads to RMR CCSD(T). The resulting energies are compared with those yielded by the standard single‐reference (SR) CCSD and CCSD(T) approaches. We first determine the optimal geometry for each species considered. Using the CC methods just mentioned we then compute the energy of the lowest‐lying singlet and triplet states, the implied singlet‐triplet splitting, and determine the spin multiplicity of the ground state. We point out the relationship between the degree of the diradical character, the extent of quasi‐degeneracy or the MR nature of the state considered, the distance separating the radical centers and, finally, the size of the largest doubly excited cluster amplitude in the CC wave functions. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009