School of Chemistry, The University of Sydney, New South Wales 2006, Australia.
J Chem Phys. 2010 Feb 7;132(5):054105. doi: 10.1063/1.3276064.
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 29 kJ mol(-1) at the CCSD(T)/6-31G* level of theory, has a 4 kJ mol(-1) dependence on the treatment of electron correlation. The anharmonic ZPEs are consistently 0.3 kJ mol(-1) lower in energy than the harmonic ZPEs calculated at the Hartree-Fock and MP2 levels of theory, and 0.7 kJ mol(-1) lower in energy at the CCSD(T)/6-31G* level of theory. Ideally, for sub-kJ mol(-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.
热化学计算中最大的误差之一是零点能 (ZPE) 的确定。使用量子扩散蒙特卡罗方法，对使用各种方法和基组组合构建的插值势能面 (PES) 上的 SSSH 自由基的完全耦合、非谐 ZPE 和基态核波函数进行了计算。在 CCSD(T)/6-31G理论水平下，SSSH 的 ZPE 约为 29 kJ mol(-1)，与电子相关处理的依赖关系为 4 kJ mol(-1)。与 Hartree-Fock 和 MP2 理论水平下计算的谐 ZPE 相比，非谐 ZPE 的能量始终低 0.3 kJ mol(-1)，与 CCSD(T)/6-31G理论水平下的能量低 0.7 kJ mol(-1)。理想情况下，对于亚千焦摩尔(-1)的热化学精度，ZPE 应该使用尽可能大的基组的相关方法进行计算。SSSH 的基态核波函数也具有显著的方法和基组依赖性。核波函数的分析表明，尽管 SSSH 具有足够的 ZPE 使其在两个最小值之间离域，但它仍局限于单个对称等效全局最小值。作为这项工作的一部分，提出了对 Collins 及其同事的插值 PES 构建方案的修改。
