CONVERGENCE TO THE BASIS-SET LIMIT IN ABINITIO CALCULATIONS AT THE CORRELATED LEVEL ON THE WATER DIMER

The equilibrium structure and binding energy of the water dimer have been determined in ab initio quantum-mechanical calculations at the correlated level using second-order Moller Plesset theory (MP2) and coupled-electron pair theory (CEPA-1). Basis set superposition error was avoided by applying the counterpoise procedure throughout. Basis set convergence was monitored by studying not only the total interaction energy, but also the first and higher-order Hartree-Fock interaction energies, the partitioned intra and intermolecular components of the MP2 interaction energy, and the monomer dipole moments. This was done at a near equilibrium geometry for more than 20 progressively improved basis sets. The largest set was used in MP2 and CEPA-1 geometry optimizations in C(s) symmetry, keeping all intramolecular coordinates fixed, except for the donor OH length. The equilibrium geometry is found to be R(OO)=2.949 (6) angstrom, theta(a) = 55.2 (2.0)-degrees, theta(d) = 57.6 (2.0)-degrees. The donor OH bond is lengthened by 0.0060 (6) angstrom, but this has virtually no effect upon the final R(OO). The equilibrium binding energy is determined as DELTAE = -4.73 (10) kcal/mol. The CEPA dipole moment is 2.60 (10) D. The error bars on these results reflect the uncertainty due to the remaining incompleteness in the one-electron basis as well as in the treatment of electron correlation. Taking into account the vibrational effects present in experimental data, the calculated results lie within the error bars of the experimental data available to date. However, the present error bars are two to seven times tighter and so some of the experimental values lie outside the present ranges. The largest discrepancy is for DELTAE, which is difficult to determine experimentally. This finding is of importance for the modeling of water properties where empirical potentials with DELTAE ranging from -5.0 to -5.5 kcal/mol are customarily employed.