Institut für Physikalische Chemie , Johannes Gutenberg-Universität Mainz , Duesbergweg 10-14 , 55128 Mainz , Germany.
J Chem Theory Comput. 2019 Mar 12;15(3):1572-1592. doi: 10.1021/acs.jctc.8b01048. Epub 2019 Feb 19.
We present the development of a perturbative triples correction scheme for the previously reported unitary group based spin-adapted combinatoric open-shell coupled-cluster (CC) singles and doubles (COS-CCSD) approach and report on the applications of the newly developed method, termed "COS-CCSD(T)", to the calculation of hyperfine coupling (HFC) tensors for radicals consisting of hydrogen, second- and third-row elements. The COS-CCSD(T) method involves a single noniterative step with [Formula: see text] scaling of the computational cost for the calculation of triples corrections to the energy. The key feature of this development is the use of spatial semicanonical orbitals generated from standard restricted open-shell Hartree-Fock (ROHF) orbitals, which allows the unperturbed Hamiltonian operator to be defined in terms of a diagonal spin-free Fock operator. The HFC tensors are computed as a first-order property via implementation of an analytic derivative scheme. The required one-particle spin density matrix is computed by using one- and two-particle spin-free density matrices that are obtained from the analytic derivative implementation, in this way avoiding the use of any spin-dependent operator and maintaining spin adaptation of the CC wavefunction. Benchmark calculations of HFC tensors for a set of 21 radicals indicate reasonably good agreement of the COS-CCSD(T) results with experiment and a consistent improvement over the COS-CCSD method. We demonstrate that the accuracies of the isotropic hyperfine coupling constants obtained in unrestricted HF (UHF) reference based spin-orbital CCSD(T) calculations deteriorate when spin contamination in the UHF wavefunction is large, and the results may even become qualitatively incorrect when spin polarization is the driving mechanism. Within a similar noniterative perturbative treatment of triple excitations, the spin-adapted COS-CCSD(T) approach produces accurate results, thus ensuring cost-effectiveness together with reliability.
我们提出了一种微扰三重态修正方案，用于之前报道的基于幺正群的自旋自适应组合开壳耦合簇（CC）单激发和双激发（COS-CCSD）方法，并报告了新开发的方法，称为“COS-CCSD(T)”，在计算由氢、第二和第三周期元素组成的自由基的超精细耦合（HFC）张量方面的应用。COS-CCSD(T)方法涉及单个非迭代步骤，对于计算能量的三重态修正的计算成本具有[Formula: see text]的比例缩放。这一发展的关键特点是使用从标准限制开壳 Hartree-Fock（ROHF）轨道生成的空间半正则轨道，这允许未扰动哈密顿算子用无自旋自由 Fock 算子来定义。HFC 张量是通过实现分析导数方案作为一阶性质来计算的。所需的单粒子自旋密度矩阵是通过使用从分析导数实现中获得的单粒子和双粒子无自旋密度矩阵来计算的，这样就避免了使用任何自旋相关算子，并保持了 CC 波函数的自旋适应性。一组 21 个自由基的 HFC 张量基准计算表明，COS-CCSD(T)结果与实验相当吻合，并且比 COS-CCSD 方法有一致的改进。我们证明，在 UHF 波函数中存在较大自旋污染时，基于无限制 HF（UHF）参考的自旋轨道 CCSD(T)计算得到的各向同性超精细耦合常数的精度会恶化，并且当自旋极化是驱动机制时，结果甚至可能变得定性不正确。在三重态激发的类似非迭代微扰处理中，自旋自适应 COS-CCSD(T)方法产生准确的结果，从而确保了成本效益和可靠性。
