  The (T) and [T] perturbative corrections are derived for multicomponent
coupled-cluster theory with single and double excitations (CCSD). Benchmarking
shows that multicomponent CCSD methods that include the perturbative
corrections are more accurate than multicomponent CCSD for the calculation of
proton affinities and absolute energies. An approximation is introduced that
includes only (T) or [T] contributions from mixed electron-nuclear excitations,
which significantly reduces computational effort with only small changes in
protonic properties.

Chemical Physics