Ab initio potential energy surface for the HCl dimer

An intermolecular potential energy surface for the dimer of hydrogen chloride in the ground state is calculated at the levels of the second-order (MP2) and fourth-order (MP4) Mo/ller–Plesset approximations using a large basis set containing bond functions. The surface is characterized by the minimum energy pathway through two equivalent hydrogen-bonded structures. The hydrogen-bonded equilibrium geometry has the centers of mass distance Rm=3.78 Å and polar angles θ1=8.0° and θ2=90.0° (at MP2 level). The well depth at the hydrogen-bonded minimum is Vm=−710.9 cm−1 at MP2 and Vm=−643.9 cm−1 at MP4 level. The interchange barrier between the two equivalent minima occurs at R=3.68 Å, θ1=θ2=46.0°, with the barrier height of 58.6 cm−1 at MP2 and 45.9 cm−1 at MP4 level (with the MP2 geometries). These results are in good agreement with a new empirical potential of Elrod and Saykally. Our calculations show that the bonding in the HCl dimer is dominated by the dispersion forces, which is different from the bonding in other classical hydrogen-bonded systems such as the hydrogen fluoride dimer and the water dimer.