The Coupled Cluster (CC) method is used to compute the electronic correlation energy in atoms and molecules and often leads to highly accurate results. However, due to its single-reference nature, standard CC in its projected form fails to describe quantum states characterized by strong electronic correlations and multi-reference projective methods become necessary. On the other hand, quantum algorithms for the solution of many-electron problems have also emerged recently. The quantum unitary variant of CC (UCC) with singles and doubles (q-UCCSD) is a popular wavefunction Ansatz for the variational quantum eigensolver algorithm. The variational nature of this approach can lead to significant advantages compared to its classical equivalent in the projected form, in particular, for the description of strong electronic correlation. However, due to the large number of gate operations required in q-UCCSD, approximations need to be introduced in order to make this approach implementable in a state-of-the-art quantum computer. In this work, we evaluate several variants of the standard q-UCCSD Ansatz in which only a subset of excitations is included. In particular, we investigate the singlet and pair q-UCCD approaches combined with orbital optimization. We show that these approaches can capture the dissociation/distortion profiles of challenging systems, such as H₄, H₂O, and N₂ molecules, as well as the one-dimensional periodic Fermi–Hubbard chain. These results promote the future use of q-UCC methods for the solution of challenging electronic structure problems in quantum chemistry.

中文翻译：

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在强相关状态下的量子轨道优化unit耦合簇方法：量子算法是否能胜过其经典等效方法？

耦合簇（CC）方法用于计算原子和分子中的电子相关能量，通常可以得到高度准确的结果。然而，由于其单参考性质，其投影形式的标准CC无法描述以强电子相关性为特征的量子态，因此多参考投影方法变得必要。另一方面，最近也出现了用于解决多电子问题的量子算法。CC（UCC）的单双态量子量子变体（q-UCCSD）是流行的波函数Ansatz用于变分量子本征求解器算法。与投影形式的经典等效方法相比，此方法的可变性质可以带来显着的优势，尤其是在描述强电子相关性方面。但是，由于q-UCCSD中需要大量的门操作，因此需要引入近似值，以使该方法可在最新的量子计算机中实现。在这项工作中，我们评估标准的Q-UCCSD的几个变种拟设在其中只包括激励的一个子集。特别是，我们研究了结合轨道优化的单重态和成对q-UCCD方法。我们表明，这些方法可以捕获具有挑战性的系统（例如H _4）的解离/畸变曲线，H ₂ O和N ₂分子，以及一维周期性费米-哈伯德链。这些结果促进了q-UCC方法在解决量子化学中具有挑战性的电子结构问题方面的未来应用。