To achieve ground state energies a high accuracy which systematically
surpasses the acccuracy MP2 and DFT calculations for reaction and
binding energies, the CCSD model should be combined with a
perturbative correction for connected triples.
The recommended approach for the correction is the CCSD(T) model
ECCSD(T) = ECCSD + E(4)DT + E(5)ST
which includes the following two terms:
tμ2CCSD〈μ2|[H, T3(2)]| HF〉
tμ1CCSD〈μ2|[H, T3(2)]| HF〉
where the approximate triples amplitudes evaluated as:
t(2)aibjck = -
In the literature one also finds sometimes the approximate
triples model CCSD[T] (also denoted as CCSD+T(CCSD)), which is
obtained by adding only
E(4)DT to the CCSD energy.
Usually CCSD(T) is slightly more accurate than CCSD[T], although
for closed-shell or spin-unrestricted open-shell reference wavefunctions
the energies of both models, CCSD(T) and CCSD[T] model, are correct
through 4.th order perturbation theory. For a ROHF reference, however,
E(5)ST contributes already in 4.th order and
only the CCSD(T) model is correct through 4.th order perturbation theory.