Diffusion quantum Monte Carlo method on diradicals using single- and multi-determinant-Jastrow trial wavefunctions and different orbitals.
Diffusion quantum Monte Carlo method on diradicals using single- and multi-determinant-Jastrow trial wavefunctions and different orbitals.
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DOI：
10.1063/5.0086606
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发表时间：
2022-03
期刊：
影响因子：
--
通讯作者：
Lu Rao;Fan Wang
中科院分区：
文献类型：
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作者：
Lu Rao;Fan Wang
本文采用扩散量子蒙特卡罗（DMC）方法计算了一系列具有π2构型的有机双自由基和双原子双自由基的单重态和三重态能量。三重态的单行列式-Jastrow（SDJ）试探波函数、单重态的双行列式-Jastrow（2DJ）试探波函数和多行列式-Jastrow（MDJ）试探波函数被用于DMC计算中，使用限制开壳层B3 LYP（ROB 3LYP）轨道、完全活性空间自洽场（CASSCF）轨道、状态平均CASSCF轨道或冻结CASSCF轨道。我们的结果表明，无论是SDJ/2DJ或MDJ与ROB 3LYP轨道的DMC能量接近或低于那些与其他轨道的有机双自由基，而他们不是很敏感的双原子双自由基的轨道。此外，使用MDJ可以在一定程度上降低DMC能量的大多数所研究的有机双自由基和一些双原子双自由基。MDJ对DMC能量的重要性可以根据CASCI波函数中主要决定因素的百分比来估计。另一方面，单重态-三重态能隙可以合理地计算与DMC使用MDJ与小于2千卡/摩尔的平均绝对误差与所有这些轨道。在实际DMC计算中，使用密度泛函理论轨道的CASCI波函数在构造MDJ试验波函数时是优选的，因为它比CASSCF方法更容易获得这样的波函数。
In this work, the diffusion quantum Monte Carlo (DMC) method is employed to calculate the energies of singlet and triplet states for a series of organic diradicals and diatomic diradicals with π2 configuration. Single-determinant-Jastrow (SDJ) trial wavefunctions for triplet states, two-determinant-Jastrow (2DJ) trial wavefunctions for the singlet states, and multi-determinant-Jastrow (MDJ) trial wavefunctions are employed in DMC calculations using restricted open-shell B3LYP (ROB3LYP) orbitals, complete-active-space self-consistent field (CASSCF) orbitals, state-average CASSCF orbitals, or frozen-CASSCF orbitals. Our results show that DMC energies using either SDJ/2DJ or MDJ with ROB3LYP orbitals are close to or lower than those with the other orbitals for organic diradicals, while they are not very sensitive to the employed orbitals for diatomic diradicals. Furthermore, using MDJ can reduce DMC energies to some extent for most of the investigated organic diradicals and some diatomic diradicals. The importance of MDJ on DMC energies can be estimated based on the percentage of main determinants in the CASCI wavefunction. On the other hand, singlet-triplet gaps can be calculated reasonably with DMC using MDJ with a mean absolute error of less than 2 kcal/mol with all these orbitals. CASCI wavefunctions using density functional theory orbitals are preferred in constructing MDJ trial wavefunctions in practical DMC calculations since it is easier to obtain such wavefunctions than CASSCF methods.