采用DMRG参考波函数的外收缩多参考组态相互作用方法
Externally-Contracted Multi-Reference Configuration Interaction Method Using a DMRG Reference Wave Function
摘要：
密度矩阵重整化群（DMRG）方法在多参考量子化学中的最新发展使得在大的活性空间中计算静态关联变得实用，而动态关联则为强关联体系的DMRG参考提供了一个关键的修正，并且通常使用多参考微扰（MRPT）或组态相互作用（MRCI）方法和内部收缩（ic）近似来获得。这些方法可以使用活动空间，可扩展到比以前可能的相对更大的大小引用。然而，由于高阶约化密度矩阵（RDM）的高计算和存储成本以及虚拟轨道的数目通常限于几百个，它们仍然很难适用于具有大于30个轨道的有效空间的系统。在这项工作中，我们提出了一种新的有效的实施DMRG-MRCI，其中我们使用重构CASCI型配置从DMRG波函数通过熵驱动遗传算法（EDGA），并集成与MRCI的外部收缩（ec）计划。该方法克服了传统DMRG动态关联方法中采用ic近似计算高阶RDM的瓶颈，并且MRCI组态的数目不依赖于虚轨道的数目.因此，DMRG-ec-MRCI方法在处理大于30个轨道和大基组的活动空间时是有希望的。我们证明了我们的DMRG-ec-MRCI方法在几个基准应用中的能力，包括Cr $_{2}$的势能曲线的评估，单三重态间隙的高n-并苯分子和Eu-BTBP（NO $_3$）$_3$复杂的能量。
英文摘要：
The recent development of the density matrix renormalization group (DMRG) method in multireference quantum chemistry makes it practical to evaluate static correlation in a large active space, while dynamic correlation provides a critical correction to the DMRG reference for strong-correlated systems and is usually obtained using multi-reference perturbation (MRPT) or configuration interaction (MRCI) methods with internal contraction (ic) approximation. These methods can use active space scalable to relatively larger size references than has previously been possible. However, they are still hardly applicable to systems with active space larger than 30 orbitals because of high computation and storage costs of high-order reduced density matrices (RDMs) and the number of virtual orbitals are normally limited to few hundreds. In this work, we propose a new effective implementation of DMRG-MRCI, in which we use re-constructed CASCI-type configurations from DMRG wave function via the entropy-driving genetic algorithm (EDGA), and integrate with MRCI by an external contraction (ec) scheme. This bypasses the bottleneck of computing high-order RDMs in traditional DMRG dynamic correlation methods with ic approximation and the number of MRCI configurations is not dependent on the number of virtual orbitals. Therefore, DMRG-ec-MRCI method is promising for dealing with larger active space than 30 orbitals and large basis sets. We demonstrate the capability of our DMRG-ec-MRCI method in several benchmark applications, including the evaluation of potential energy curve of Cr$_{2}$, single-triplet gaps of higher n-acene molecules and the energy of Eu-BTBP(NO$_3$)$_3$ complex.
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