Zusammenfassung
The formalism is presented for the linear response of a time-dependent (TD) variational coupled cluster (VCC), truncated according to Moller-Plesset perturbation theory, i.e., a TD-VCC[n] linear response, where n denotes the order of the corresponding quasienergy with respect to the fluctuation potential. The resulting eigenvalue problem determining the excitation energies is Hermitian and of the ...
Zusammenfassung
The formalism is presented for the linear response of a time-dependent (TD) variational coupled cluster (VCC), truncated according to Moller-Plesset perturbation theory, i.e., a TD-VCC[n] linear response, where n denotes the order of the corresponding quasienergy with respect to the fluctuation potential. The resulting eigenvalue problem determining the excitation energies is Hermitian and of the simple Tamm-Dancoff form. The VCC[1] excitation energies are equivalent to those of the configuration-interaction singles (CIS) model, while the Casida equation for the TD-Hartree-Fock approach is an approximation to it. The TD-VCC[2] response, the lowest-order method including electron correlation, is discussed in detail and the relations to other second-order methods, such as the CC2 linear response and the algebraic diagrammatic construction at second order [ADC(2)] are explored.
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