Department of Chemistry, Lensfield Road, Cambridge CB2 1EW, United Kingdom.
J Chem Phys. 2019 Nov 7;151(17):174103. doi: 10.1063/1.5123146.
We present an approach to combining selected configuration interaction (SCI) and initiator full configuration interaction quantum Monte Carlo (i-FCIQMC). In the current i-FCIQMC scheme, the space of initiators is chosen dynamically by a population threshold. Here, we instead choose initiators as the selected space (V) from a prior SCI calculation, allowing substantially larger initiator spaces for a given walker population. While SCI+PT2 adds a perturbative correction in the first-order interacting space beyond V, the approach presented here allows a variational calculation in the same space and a perturbative correction in the second-order interacting space. The use of a fixed initiator space reintroduces population plateaus into FCIQMC, but it is shown that the plateau height is typically only a small multiple of the size of V. Thus, for a comparable fundamental memory cost to SCI+PT2, a substantially larger space can be sampled. The resulting method can be seen as a complementary approach to SCI+PT2, which is more accurate but slower for a common selected/initiator space. More generally, our results show that approaches exist to significantly improve initiator energies in i-FCIQMC while still ameliorating the fermion sign problem relative to the original FCIQMC method.
我们提出了一种将选定组态相互作用（SCI）和引发剂全组态相互作用量子蒙特卡罗（i-FCIQMC）相结合的方法。在当前的 i-FCIQMC 方案中，引发剂的空间是通过种群阈值动态选择的。在这里，我们选择了从先前的 SCI 计算中选择引发剂作为选定空间（V），从而为给定的 walker 种群提供了更大的引发剂空间。虽然 SCI+PT2 在 V 之外的第一相互作用空间中添加了微扰校正，但这里提出的方法允许在相同空间中进行变分计算和第二相互作用空间中的微扰校正。使用固定的引发剂空间会将种群高原重新引入到 FCIQMC 中，但结果表明高原高度通常仅为 V 的一小部分。因此，与 SCI+PT2 相比，对于可比的基本内存成本，可以对更大的空间进行采样。所得到的方法可以看作是 SCI+PT2 的补充方法，对于常见的选定/引发剂空间，它更准确但速度较慢。更一般地，我们的结果表明，存在一些方法可以显著提高 i-FCIQMC 中的引发剂能量，同时相对于原始 FCIQMC 方法改善费米子符号问题。
