Musiał Monika, Bartlett Rodney J
Quantum Theory Project, Department of Chemistry and Physics, University of Florida, Gainesville, Florida 32611, USA.
J Chem Phys. 2005 Jun 8;122(22):224102. doi: 10.1063/1.1926273.
To assess the limits of single-reference coupled-cluster (CC) methods for potential-energy surfaces, several methods have been considered for the inclusion of connected quadruple excitations. Most are based upon the factorized inclusion of the connected quadruple contribution (Qf) [J. Chem. Phys. 108, 9221 (1998)]. We compare the methods for the treatment of potential-energy curves for small molecules. These include CCSD(TQf), where the initial contributions of triple (T) and factorized quadruple excitations are added to coupled-cluster singles (S) and doubles (D), its generalization to CCSD(TQf), where instead of measuring their first contribution from orders in H, it is measured from orders in H=e(-(T1+T2))He(T1+T2); renormalized approximations of both, and CCSD2 defined in [J. Chem. Phys. 115, 2014 (2001)]. We also consider CCSDT, CCSDT(Qf), CCSDTQ, and CCSDTQP for comparison, where T, Q, and P indicate full triple, quadruple, and pentuple excitations, respectively. Illustrations for F2, the double bond breaking in water, and N2 are shown, including effects of quadruples on equilibrium geometries and vibrational frequencies. Despite the fact that no perturbative approximation, as opposed to an iterative approximation, should be able to separate a molecule correctly for a restricted-Hartree-Fock reference function, some of these higher-order approximations have a role to play in developing new, more robust procedures.
为了评估单参考耦合簇（CC）方法在势能面方面的局限性，人们考虑了几种纳入连接四重激发的方法。大多数方法基于连接四重贡献（Qf）的因式分解纳入[《化学物理杂志》108, 9221 (1998)]。我们比较了处理小分子势能曲线的方法。这些方法包括CCSD(TQf)，即将三重激发（T）和因式分解的四重激发的初始贡献加到耦合簇单激发（S）和双激发（D）上，其推广形式CCSD(TQf)，其中不是从哈密顿量（H）的阶次来衡量它们的首次贡献，而是从H = e^(-(T1 + T2))He^(T1 + T2)的阶次来衡量；两者的重整化近似，以及在[《化学物理杂志》115, 2014 (2001)]中定义的CCSD2。我们还考虑了CCSDT、CCSDT(Qf)、CCSDTQ和CCSDTQP用于比较，其中T、Q和P分别表示完整的三重、四重和五重激发。展示了F2、水中双键断裂以及N2的示例，包括四重激发对平衡几何结构和振动频率的影响。尽管与迭代近似相反，没有微扰近似应该能够针对受限哈特里 - 福克参考函数正确分离分子，但这些高阶近似中的一些在开发新的、更稳健的程序中仍有作用。
