Dynamical correlation energy of metals in large basis sets from downfolding and composite approaches
Dynamical correlation energy of metals in large basis sets from downfolding and composite approaches
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DOI：
10.1063/5.0049890
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发表时间：
2021-06-07
影响因子：
4.4
通讯作者：
Berkelbach, Timothy C.
中科院分区：
文献类型：
--
作者：
Callahan, James M.;Lange, Malte F.;Berkelbach, Timothy C.
单双激发耦合团簇理论（CCSD）是一种很有前途的三维金属电子结构的从头计算方法，但二阶微扰理论（MP2）在热力学极限上存在分歧。然而，由于成本高，收敛性差的CCSD的基规模方面，CCSD应用于周期系统往往会导致大的基集误差。在一个常见的“复合”方法中，MP2被用来通过焦点校正来恢复丢失的动力学相关能，但金属的有限阶微扰理论的不足引起了对这种方法的质疑。在这里，我们描述了如何高能量的激发处理MP2可以“向下折叠”到一个低能量的活动空间CCSD治疗。比较复合和向下折叠的方法如何执行均匀的电子气，我们发现，后者收敛更快的基组大小。尽管如此，复合方法是令人惊讶的准确，因为它消除了有问题的MP2处理费米表面附近的双激发。用这种方法在完全基组和热力学极限下估计CCSD的相关能，发现在r（s）= 4时，CCSD能恢复85%-90%的精确相关能.我们还使用直接随机相位近似代替MP2来测试复合方法，由于需要包含在更昂贵的CCSD计算中的轨道数量较少，因此通常（但不总是）更具有成本效益。
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 finite-order perturbation theory 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 this method to estimate the CCSD correlation energy in the combined complete basis set and thermodynamic limits, we find that CCSD recovers 85%-90% of the exact correlation energy at r(s) = 4. We also test the composite approach with the direct random-phase approximation used in place of MP2, yielding a method that is typically (but not always) more cost effective due to the smaller number of orbitals that need to be included in the more expensive CCSD calculation.