A new potential energy surface for the water dimer obtained from separate fits of ab initio electrostatic, induction, dispersion and exchange energy contributions†

Abstract

A new potential energy surface (PES) for the water dimer based on the fit of analytical expressions to individual contributions to the total interaction energy is presented. These contributions are the first-order electrostatic energy, the first-order exchange energy, the second-order and higher-order induction plus exchange-induction energy, and finally the second-order dispersion plus exchange-dispersion energy, as obtained from ab initio symmetry-adapted perturbation theory (SAPT) using an augmented triple-zeta basis set including bond functions. The new model potential is derived from nearly 500 systematically chosen dimer geometries, employing a rigid vibrationally averaged monomer geometry. All SAPT total interaction energies are compared with those from complete basis set-extrapolated supermolecular calculations employing second-order Møller–Plesset (MP2) and coupled-cluster theory including up to non-iterative triple excitations (CCSD(T)). Due to a fortunate, but fairly systematic cancellation of errors the SAPT interaction energies agree slightly better with CCSD(T) than does MP2. The new model potential reproduces the stationary points on the PES as known from ab initio calculations very well, while the fully quantum-corrected second virial coefficient is about 14% too low in the temperature range of 400–850 K. A second analytical PES differing in the expression for the first-order exchange energy yields a better virial coefficient, but fails to reproduce the correct minimum structure of the water dimer. †Dedicated to Andrzej Sadlej on the occasion of his 65th birthday.