Anisotropic intermolecular interactions in van der Waals and hydrogen-bonded complexes: What can we get from density functional calculations?

The applicability of various density functional theory (DFT) methods to des cribe the anisotropy of the intermolecular potential energy surfaces of hyd rogen-bonded [OH--H2O, (H2O)(2)] and van der Waals [CO-H2O, He-CO2] complex es has been tested by comparison with supermolecule CCSD(T) (coupled-cluste r method restricted to single, double, and noniterative triple excitations) and perturbational SAPT (symmetry-adapted perturbation theory) results com puted for the same geometries and with the same basis sets. It is shown tha t for strongly bound ionic hydrogen-bonded complexes, like OH--H2O, hybrid approaches provide accurate results. For other systems, including the water dimer, the DFT calculations fail to reproduce the correct angular dependen ce of the potential surfaces. It is also shown that a hybrid functional adj usted to reproduce the CCSD(T) value of the binding energy for the water di mer produces results worse than the standard hybrid functionals for OH--H2O , and fails to describe the correct anisotropy of the CO-H2O interaction. ( C) 1999 American Institute of Physics. [S0021-9606(99)30841-2].