Method and basis set dependence of anharmonic ground state nuclear wave functions and zero-point energies: Application to SSSH

One of the largest remaining errors in thermochemical calculations is the determination of the zero-point energy (ZPE). The fully coupled, anharmonic ZPE and ground state nuclear wave function of the SSSH radical are calculated using quantum diffusion Monte Carlo on interpolated potential energy surfaces (PESs) constructed using a variety of method and basis set combinations. The ZPE of SSSH, which is approximately $29kJmol−1$ at the $CCSD(T)/6-31G∗$ level of theory, has a $4kJmol−1$ dependence on the treatment of electron correlation. The anharmonic ZPEs are consistently $0.3kJmol−1$ lower in energy than the harmonic ZPEs calculated at the Hartree–Fock and MP2 levels of theory, and $0.7kJmol−1$ lower in energy at the $CCSD(T)/6-31G∗$ level of theory. Ideally, for sub-$kJmol−1$ thermochemical accuracy, ZPEs should be calculated using correlated methods with as big a basis set as practicable. The ground state nuclear wave function of SSSH also has significant method and basis set dependence. The analysis of the nuclear wave function indicates that SSSH is localized to a single symmetry equivalent global minimum, despite having sufficient ZPE to be delocalized over both minima. As part of this work, modifications to the interpolated PES construction scheme of Collins and co-workers are presented.