量子Monte Carlo和密度泛函理论对小团簇能量学的准确性评价
Assessing the accuracy of quantum Monte Carlo and density functional theory for energetics of small water clusters
摘要：
我们提出了一个详细的研究的能量水团簇（H $_2$ O）$_n$与$n \le 6$，比较扩散蒙特卡罗（DMC）和近似密度泛函理论（DFT）以及收敛耦合集群基准。我们利用总能量的多体分解将DMC和DFT的误差分为1体、2体和超2体分量。使用平衡集群配置和热合奏的配置，我们发现DMC是一致的更准确的比DFT，部分原因是一些近似泛函给穷人1-体畸变能量。即使当这些被校正时，DFT仍然比DMC精确得多。当DFT的1-和2-体误差被校正时，一些泛函在精度上与DMC竞争;然而，其他泛函仍然更差，表明它们遭受显著的超2-体误差。结合这里提出的证据与最近证明的DMC冰结构的高精度，我们建议如何DMC现在可以用来提供更大的集群和散装液态水的基准。
英文摘要：
We present a detailed study of the energetics of water clusters (H$_2$O)$_n$ with $n \le 6$, comparing diffusion Monte Carlo (DMC) and approximate density functional theory (DFT) with well converged coupled-cluster benchmarks. We use the many-body decomposition of the total energy to classify the errors of DMC and DFT into 1-body, 2-body and beyond-2-body components. Using both equilibrium cluster configurations and thermal ensembles of configurations, we find DMC to be uniformly much more accurate than DFT, partly because some of the approximate functionals give poor 1-body distortion energies. Even when these are corrected, DFT remains considerably less accurate than DMC. When both 1- and 2-body errors of DFT are corrected, some functionals compete in accuracy with DMC; however, other functionals remain worse, showing that they suffer from significant beyond-2-body errors. Combining the evidence presented here with the recently demonstrated high accuracy of DMC for ice structures, we suggest how DMC can now be used to provide benchmarks for larger clusters and for bulk liquid water.
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