The electron pair approximation offers an efficient variational quantum eigensolver (VQE) approach for chemistry simulations on quantum computers. With the number of entangling gates scaling quadratically with system size and a constant measurement overhead, the orbital optimized unitary pair coupled cluster double (oo-upCCD) ansatz strikes a balance between accuracy and efficiency. However, the electron pair approximation prevents the method from achieving quantitative accuracy. To improve it, we explore the theory of second order perturbation (PT2) correction to oo-upCCD. PT2 accounts for the missing broken-pair contributions in oo-upCCD, while retaining its efficiencies. For molecular bond stretching and chemical reactions, the method significantly improves the predicted energy accuracy, reducing oo-upCCD’s error by up to 90%. On IonQ’s quantum computers, we find that the PT2 energy correction is highly noise-resilient. The predicted VQE-PT2 reaction energies are in excellent agreement with noise-free simulators after applying simple error mitigations solely on the VQE energies.

电子对近似为量子计算机上的化学模拟提供了一种有效的变分量子本征求解器 (VQE) 方法。由于纠缠门的数量与系统尺寸呈二次方关系以及恒定的测量开销，轨道优化酉对耦合簇双 (oo-upCCD) ansatz 在精度和效率之间取得了平衡。然而，电子对近似阻碍了该方法实现定量精度。为了改进它，我们探索了对 oo-upCCD 的二阶微扰（PT2）校正理论。 PT2 解释了 oo-upCCD 中缺失的断对贡献，同时保留其效率。对于分子键拉伸和化学反应，该方法显着提高了预测能量的准确性，将 oo-upCCD 的误差降低高达 90%。在 IonQ 的量子计算机上，我们发现 PT2 能量校正具有很高的抗噪声能力。仅对 VQE 能量应用简单的误差缓解后，预测的 VQE-PT2 反应能与无噪声模拟器非常一致。