Adv. Quant. Chem., 50, 37-60 (2005).
Available online 15 November 2005
The performance of the second-order methods for excitation energies CC2
and ADC(2) is investigated and compared with the more approximate CIS
and CIS(D) methods as well as with the coupled-cluster models CCSD,
CCSDR(3) and CC3. As a by-product of this investigation the first
implementation of analytic excited state gradients for ADC(2) and
CIS(D,,) is reported.
It is found that for equilibrium structures and vibrational frequencies
the second-order models CIS(D), ADC(2) and CC2 give often results close
to those obtained with CCSD. The main advantage of CCSD lies in its
robustness with respect to strong correlation effects. For adiabatic
excitation energies CC2 is found to give from all second-order methods
for excitation energies (including CCSD) the smallest mean absolute
errors. ADC(2) and CIS(D 0) are found to give almost identical results.
An advantage of ADC(2) compared to CC2 is that the excitation energies
are obtained as eigenvalues of a Hermitian secular matrix, while in
coupled-cluster response the excitation energies are obtained as
eigenvalues of a non-Hermitian Jacobi matrix. It is shown that, as a
consequence of the lack of Hermitian symmetry, the latter methods will
in general not give a physically correct description of conical
intersections between states of the same symmetry. This problem does
not appear in ADC(2).
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