π-Electron Calculations Using the Local Linear-Response Coupled-Cluster Singles and Doubles Theory

Abstract

Calculations of electronic excitation energies, electron charge distribution, and optical parameters for π-conjugated oligomeric systems are carried out with a new local semiempirical π-electron variant of the coupled clusters (CC) theory involving single and double excitations. The results obtained in the calculations demonstrate adequate accuracy of the proposed linear-response generalization of the local semiempirical π-electron CC approach employing covalently nonbonded molecules of ethylene as the basis set in the calculation. Limit values corresponding to the infinite system size for the lowest excitation energies for polyenes and for condensed and noncondensed polyfulvenes are obtained.

Electronic perturbation effects in the presence of electric field for π‐conjugated systems: An electron‐correlation study

“…For most calculation of AAP the coupled cluster (CC) method with account for single and double excitations (CCSD) (see for instance [29,30]) is employed. The method is implemented in the author's program package HELIOS [25][26][27]. It is worth mentioning that the interest in the π-electron calculations in the framework of the CC theory has a long history dating back to the first works of Čížek and Paldus [31,32,33] and continuing up to the present day [25][26][27][28]34,35].…”

“…The hierarchy of approximations based on (5) for performing CCSD calculations of long (oligomer) π-systems was proposed in references [25][26][27][28]. In the expressions ( 6) and ( 7) the Hamiltonian, Н, includes one-and two-electron contributions ( 0 H ) in accordance with the Pariser-Parr-Pople method [37,38].…”

“…As computation experience shows [25][26][27][28]53], calculations of most electronic susceptibilities of π-electrons systems require an accurate account of the electron correlation effects. Thus, the first question that needs to be answered is how important these effects are in calculation of AAP.…”

“…Detailed investigations of electronic properties of π-systems show the importance of the electron correlations effects in determining lower-and higher-order polarizabilities. See for instance Refs [25][26][27][28] investigating different π-systems (polyenes, polyynes, non-alternant hydrocarbons, polycyclic oligomer chains).…”