Which isomeric form of formaldehyde dimer is the most stable – a high-level coupled-cluster study

Frozen-core and all-electron CCSD(T) calculations have been performed in order to derive accurate geometrical characteristics as well as dimerization energies of the two lowest-lying isomers of formaldehyde dimer (H2CO)2: dimer I (Cs) and dimer II (C2h). Contrary to early MP2 calculations, it has been unambiguously determined on the basis of CCSD(T) complete-basis-set extrapolations that dimer I is the true global-minimum structure, which lies 0.3–0.4 kcal/mol lower in energy than the dimer II structure at 0 K. The obtained equilibrium geometries and dimerization energies can serve as a benchmark for testing the performance of other, less computationally demanding methods.