标题： 金属在大基组中的动态关联能从降维和组合方法中获得 显示英文标题

标题： Dynamical correlation energy of metals in large basis sets from downfolding and composite approaches

摘要： 耦合簇理论包含单激发和双激发（CCSD）是一种用于三维金属电子结构的有前途的第一性原理方法，对于这种体系，二阶微扰理论（MP2）在热力学极限下会发散。 然而，由于CCSD对基组大小的高成本和收敛性差，将其应用于周期性系统通常会导致大的基组误差。 在一种常见的“组合”方法中，通过焦点点校正使用MP2来恢复缺失的动力学相关能，但MP2在金属中的不足引发了对该方法的质疑。 在这里，我们描述了如何将MP2处理的高能激发“下折叠”到一个低能活性空间，由CCSD进行处理。 比较组合方法和下折叠方法在均匀电子气中的表现，我们发现后者在基组大小方面收敛得更快。 尽管如此，组合方法出人意料地准确，因为它消除了MP2对费米面附近双激发的有问题的处理。 使用该方法估计在结合完全基组和热力学极限下的CCSD相关能，我们发现CCSD在$r_s=4$时恢复了超过90%的精确相关能。 我们还用随机相位近似代替MP2测试了组合和下折叠方法，得到一种更有效但更昂贵的方法。 显示英文摘要

摘要： Coupled-cluster theory with single and double excitations (CCSD) is a promising ab initio method for the electronic structure of three-dimensional metals, for which second-order perturbation theory (MP2) diverges in the thermodynamic limit. However, due to the high cost and poor convergence of CCSD with respect to basis size, applying CCSD to periodic systems often leads to large basis set errors. In a common "composite" method, MP2 is used to recover the missing dynamical correlation energy through a focal-point correction, but the inadequacy of MP2 for metals raises questions about this approach. Here we describe how high-energy excitations treated by MP2 can be "downfolded" into a low-energy active space to be treated by CCSD. Comparing how the composite and downfolding approaches perform for the uniform electron gas, we find that the latter converges more quickly with respect to the basis set size. Nonetheless, the composite approach is surprisingly accurate because it removes the problematic MP2 treatment of double excitations near the Fermi surface. Using the method to estimate the CCSD correlation energy in the combined complete basis set and thermodynamic limits, we find CCSD recovers over 90% of the exact correlation energy at $r_s=4$. We also test the composite and downfolding approaches with the random-phase approximation used in place of MP2, yielding a method that is more effective but more expensive.