Comparison of CCSDT-n methods with coupled-cluster theory with single and double excitations and coupled-cluster theory with single, double, and triple excitations in terms of many-body perturbation theory - what is the mosteffective triple-excitation method?
Y. He et al., Comparison of CCSDT-n methods with coupled-cluster theory with single and double excitations and coupled-cluster theory with single, double, and triple excitations in terms of many-body perturbation theory - what is the mosteffective triple-excitation method?, THEOR CH AC, 105(3), 2001, pp. 182-196
CCSDT-n methods (n = 1, ..., 4) are analyzed in terms of many-body perturba
tion theory and compared with CCSD, CCSD(T), and CCSDT. Correlation terms,
which are introduced at each CCSDT-n level are identified and discussed wit
h regard to their importance for the total energy. CCSDT-la represents the
strongest improvement of CCSD since it covers 75% of the T terms added at t
he CCSDT level. At CCSDT-2 and CCSDT-3, only partial energy terms are added
where the positive ..TQ.. coupling terms at CCSDT-2 are more important tha
n the slightly negative ..TTS terms at CCSDT-3. CCSDT-4 leads to new ..TT..
coupling effects, which lower the energy. Calculated CCSDT-n/correlation-c
onsistent polarized-valence triple-zeta spectra reveal that the energy chan
ges caused by CCSDT-2, CCSDT-3, CCSDT-4. and CCSDT largely cancel each othe
r out; however, because of a dependence of these contributions on electron
clustering the importance of these effects for the correct description of c
ertain chemical reactions must be considered. It is shown that CCSDT-1 las
well as all other CCSDT-n) calculations lead to atomization energies as acc
urate as G2 values, where this result reflects the importance of including
at least some of the T effects.