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 (FOIS) 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.
Boosting the accuracy and speed of quantum Monte Carlo: size-consistency and time-step
扩散蒙特卡罗（DMC）模拟费米子正在成为标准，以提供高质量的参考数据的系统，太大，无法通过量子化学方法进行研究。DMC与固定节点近似依赖于修改的格林函数，以避免奇异性附近的节点表面的试验波函数。我们发现，这些修改影响DMC能量的方式是不大小一致的，导致大的时间步长误差。基于Umrigar { \em et al.}的修改，以及DePasquale { \em et al.}我们提出了一个简单的格林函数修改，恢复大小的一致性，以大的时间步长值，大大减少时间步长的错误。新的算法也产生显着的加速高达两个数量级的分子-分子结合能和晶体内聚能的计算，从而扩展了什么是可能的DMC的视野。
