激发态的无相位相干场量子蒙特卡罗计算：低激发态C2的potential energy曲线
Excited state calculations using phaseless auxiliary-field quantum Monte Carlo: potential energy curves of low lying C2 singlet states
摘要：
我们表明，最近开发的无相辅助场量子蒙特卡罗（AFQMC）方法可用于研究激发态，为标准量子化学方法提供了一种替代方案。无相AFQMC方法的计算成本与系统尺寸M成M^3-M^4比例，已被证明是基态计算中最精确的多体方法之一。对于激发态，通过引入试探波函数的近似无相约束，实现了对塌缩到基态的防止和对费米子符号/相位问题的控制。以具有挑战性的C2分子为例，计算了基态和两个低单重态激发态的势能曲线。试验波函数是通过截断完整的有源空间波函数而获得的，没有进一步的优化。使用小基组的无相位AFQMC结果与精确的全组态相互作用计算结果吻合良好，而使用大基组的结果与实验光谱常数吻合良好。
英文摘要：
We show that the recently developed phaseless auxiliary-field quantum Monte Carlo (AFQMC) method can be used to study excited states, providing an alternative to standard quantum chemistry methods. The phaseless AFQMC approach, whose computational cost scales as M^3-M^4 with system size M, has been shown to be among the most accurate many-body methods in ground state calculations. For excited states, prevention of collapse into the ground state and control of the Fermion sign/phase problem are accomplished by the approximate phaseless constraint with a trial wave function. Using the challenging C2 molecule as a test case, we calculate the potential energy curves of the ground and two low-lying singlet excited states. The trial wave function is obtained by truncating complete active space wave functions, with no further optimization. The phaseless AFQMC results using a small basis set are in good agreement with exact full configuration interaction calculations, while those using large basis sets are in good agreement with experimental spectroscopic constants.
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