What Function Is Optimal for CCSD(T) Complete Basis Extrapolation for OH Stretching Potential of Water Molecules?

摘要

A procedure to calculate the complete basis set limits of electronic energy of water molecules is developed based on two levels of extrapolation scheme. A three-point fitting scheme with the energies computed at the levels of basis function, aug-cc-pVXZ for X = 4-6, is employed to obtain accurate energies of a monomer and a dimer of water. Three extrapolation functions considered in this article are found to work excellently to give accurate Hartree-Fock (HF) energies of water molecules. However, the three functions give slightly different values up to 1-2 mHartree for the complete basis limit of electronic correlation energy. The best performing extrapolating function for the five-point fitting scheme with a larger set of basis functions for X = 2-6 has been determined aiming for the extrapolation from the lowest two levels of basis sets, double and triple zeta basis functions. For this purpose, an exponential function is suggested for HF energy while an exponential function with a square root exponent is suggested for electronic correlation energy. The optimal exponents of the two functions have been determined.