A procedure to calculate the complete basis set limits of electronic energy of water molecules is developed based on two levels of fitting 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. To represent the convergence pattern of the three levels of energy, we consider three extrapolation functions, power, stretched exponential, and ordinary exponential functions. The three functions 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. Although the correlation energy plays a critical role in determining the potential energy curve of OH stretching, the energetic difference with respect to the type of extrapolation function are found not to be reflected in the potential curve. Aiming for applying to a mixed quantum/classical theory, we found a strong correlation between the extrapolated energy and the energies calculated at the level of double and triple zeta basis functions. This enables us to reproduce the extrapolated energy from the latter energies with much cheaper computation.

Quantum chemical extrapolation scheme to calculate potential energy of OH stretching in water

256th National Meeting (2018)

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