  The benzene...ethene and parallel-displaced (PD) benzene...benzene dimers are
the most fundamental systems involving p-p stacking interactions. Several
high-level ab initio investigations calculated the binding energies of these
dimers at the CCSD(T)/CBS level of theory using various approaches such as
reduced virtual orbital spaces and/or MP2-based basis set corrections. Here we
obtain CCSDT(Q) binding energies using a Weizmann-3-type approach. In
particular, we extrapolate the SCF, CCSD, and (T) components using large
heavy-atom augmented Gaussian basis sets (namely, SCF/jul-cc-pV{5,6}Z,
CCSD/jul-cc-pV{Q,5}Z, and (T)/jul-cc-pV{T,Q}Z). We consider post-CCSD(T)
contributions up to CCSDT(Q), inner-shell, scalar-relativistic, and
Born-Oppenheimer corrections. Overall, our best relativistic, all-electron
CCSDT(Q) binding energies are Delta Ee,all,rel = 1.234 (benzene...ethene) and
2.550 (benzene...benzene PD), Delta H0 = 0.949 (benzene...ethene) and 2.310
(benzene...benzene PD), and Delta H298 = 0.130 (benzene...ethene) and 1.461
(benzene...benzene PD) kcal/mol. Important conclusions are reached regarding
the basis set convergence of the SCF, CCSD, (T), and post-CCSD(T) components.
Explicitly correlated calculations are used as a sanity check on the
conventional binding energies. Overall, post-CCSD(T) contributions are
destabilizing by 0.028 (benzene...ethene) and 0.058(benzene...benzene)
kcal/mol, thus they cannot be neglected if 0.1 kcal/mol accuracy is sought.
