Ma Qianli, Werner Hans-Joachim
Institut für Theoretische Chemie, Universität Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart, Germany.
J Chem Theory Comput. 2020 May 12;16(5):3135-3151. doi: 10.1021/acs.jctc.0c00192. Epub 2020 Apr 27.
We present well-parallelized local implementations of high-spin open-shell coupled cluster methods with single and double excitations (CCSD) using pair natural orbitals (PNOs). The methods are based on the spin-orbital coupled cluster theory using restricted open-shell Hartree-Fock (ROHF) reference functions. Two variants, namely, PNO-UCCSD and PNO-RCCSD are implemented and compared. In PNO-UCCSD, the coupled cluster amplitudes are spin-unrestricted, while in PNO-RCCSD the linear terms are spin-adapted by a spin-projection approach as described in 1993, 99, 5219-5227. Near linear scaling of the computational cost with the number of correlated electrons is achieved by applying domain and pair approximations. The PNOs are spin-independent and obtained using a semicanonical spin-restricted MP2 approximation with large domains of projected atomic orbitals (PAOs). The pair approximations of our previously described closed-shell PNO-LCCSD method are carefully revised so that they are compatible to the UCCSD theory, and PNO-UCCSD or PNO-RCCSD calculations for closed-shell molecules yield exactly the same results as corresponding spin-free closed-shell PNO-LCCSD calculations. The convergence of the results with respect to the thresholds and options that control the domain and pair approximations is demonstrated. It is found that large domains are required for the single excitations in open-shell calculations in order to obtain converged results. In general, the errors of relative energies caused by the local approximations can be reduced to below 1 kcal mol, even for difficult cases. Presently, PNO-RCCSD and PNO-UCCSD calculations for molecules with 100-200 atoms and augmented triple-ζ basis sets can be carried out in a few hours of elapsed time using ∼100 CPU cores. In addition, the program is also capable of performing distinguishable cluster (PNO-RDCSD and PNO-UDCSC) calculations. The present work is a critical step in developing fully local open-shell PNO-RCCSD(T)-F12 methods.
我们展示了使用对自然轨道（PNO）的高自旋开壳层耦合簇单双激发方法（CCSD）的高度并行化局部实现。这些方法基于使用受限开壳层哈特里 - 福克（ROHF）参考函数的自旋轨道耦合簇理论。实现并比较了两种变体，即PNO - UCCSD和PNO - RCCSD。在PNO - UCCSD中，耦合簇振幅是自旋非限制的，而在PNO - RCCSD中，线性项通过1993年第99卷第5219 - 5227页所述的自旋投影方法进行自旋适配。通过应用域和对近似，实现了计算成本与相关电子数的近线性缩放。PNO是自旋独立的，并且使用具有大投影原子轨道（PAO）域的半规范自旋限制MP2近似获得。我们之前描述的闭壳层PNO - LCCSD方法的对近似经过仔细修订，使其与UCCSD理论兼容，并且闭壳层分子的PNO - UCCSD或PNO - RCCSD计算产生与相应的无自旋闭壳层PNO - LCCSD计算完全相同的结果。展示了结果相对于控制域和对近似的阈值和选项的收敛性。发现在开壳层计算中，单激发需要大的域才能获得收敛结果。一般来说，即使对于困难的情况，由局部近似引起的相对能量误差也可以降低到1 kcal/mol以下。目前，使用约100个CPU核心，对于具有100 - 200个原子和增强三重ζ基组的分子，PNO - RCCSD和PNO - UCCSD计算可以在几个小时的运行时间内完成。此外，该程序还能够执行可区分簇（PNO - RDCSD和PNO - UDCSC）计算。目前的工作是开发完全局部的开壳层PNO - RCCSD(T) - F12方法的关键一步。
