Subtle basis set effects on hydrogen bonded systems

Basis sets and the theoretical treatment of electron correlation have been systematically varied to characterize subtle effects on the molecular properties of the HF monomer and dimer. Surprisingly, freezing the F 1s-like core orbitals and deleting the correspond 1s*-like virtual orbitals from the coupled-cluster correlation procedure which included all single, double and perturbatively applied triple excitations [CCSD(T)] has essentially no effect on the properties of HF and (HF)2. However, the use of pure spherical harmonic (i.e., five d and seven f) rather than Cartesian (i.e., six d and ten f) Gaussian basis functions produces a 0.22 kcal mol-1 change in the equilibrium binding energy and a 30 cm-1 change in the out-of-plane harmonic vibrational frequency of (HF)2. Discussion focuses on effects from the inclusion or exclusion of the redundant Cartesian d and f functions as well as elucidation of such aberrant behaviour. Also presented are results obtained with double-, triple-, and quadruple-zeta correlation consistent basis sets. This is the first time the harmonic vibrational frequencies of the dimer have been reported in the literature at the CCSD(T) theoretical level with the large corelation consistent quadruple-zeta basis (cc-pVQZ). This basis reproduces experimental and semi-empirical geometrical parameters for the monomer and dimer exceptionally well. However, agreement is less satisfactory between the cc-pVQZ and semi-empirical harmonic vibrational frequencies and binding energies. For the dimer the CCSD(T) geometrical parameters obtained with the cc-pVQZ basis set are: rf = 0.9194 Å, rb = 0.9217 Å, R = 2.7272 Å, θf = 71.04° and θb = 7.60°. The harmonic vibrational frequencies (,ωn) and binding energies (De and Dh0) are: ω1 = 4123 cm-1, ω2 = 4050 cm-1, ω3 = 582 cm-1, ω4 = 164 cm-1, ω5 = 219 cm-1, ω6 = 473 cm-1, De = 4.91 kcal mol-1 and Dh0 = 3.07 kcal mol-1. Smaller basis sets constructed from Dunning's 1971 F (10s6p/5s3p) work actually provide better predictions of the (HF)2 vibrational frequencies and binding energy than does the cc-pVQZ set.