消除自旋污染的量子蒙特卡罗方法：F2的真实potential energy曲线
Eliminating spin contamination in auxiliary-field quantum Monte Carlo: realistic potential energy curve of F2
摘要：
近似参考态波函数的应用|电子多体方法中的φ_r>能破坏Born-Oppenheimer自旋无关哈密顿量的自旋对称性。这可能会导致严重的错误，尤其是当粘合被拉伸或断裂时。本文介绍了一种简单的自旋投影方法，它可以在自旋污染的情况下得到无自旋污染的结果。|Φ_r>。该方法适用于困难的F2分子，这是不受限制的Hartree-Fock（UHF）。用超高频|Phi_r>，自旋污染会导致AFQMC中中间键断裂区域的大系统误差和长平衡时间。自旋投影方法消除了这些问题，并提供了一个准确的势能曲线，从平衡到解离极限使用超高频|Φ_r>。在cc-pVQZ基下得到了实际势能曲线。光谱常数的计算值与实验值符合得很好。
英文摘要：
The use of an approximate reference state wave function |Phi_r> in electronic many-body methods can break the spin symmetry of Born-Oppenheimer spin-independent Hamiltonians. This can result in significant errors, especially when bonds are stretched or broken. A simple spin-projection method is introduced for auxiliary-field quantum Monte Carlo (AFQMC) calculations, which yields spin-contamination-free results, even with a spin-contaminated |Phi_r>. The method is applied to the difficult F2 molecule, which is unbound within unrestricted Hartree-Fock (UHF). With a UHF |Phi_r>, spin contamination causes large systematic errors and long equilibration times in AFQMC in the intermediate, bond-breaking region. The spin-projection method eliminates these problems, and delivers an accurate potential energy curve from equilibrium to the dissociation limit using the UHF |Phi_r>. Realistic potential energy curves are obtained with a cc-pVQZ basis. The calculated spectroscopic constants are in excellent agreement with experiment.
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