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Vertical and adiabatic excitations in anthracene from quantum Monte Carlo: Constrained energy minimization for structural and electronic excited-state properties in the JAGP ansatz

机译:量子蒙特卡洛中蒽的垂直和绝热激发:JAGP ansatz中结构和电子激发态性质的受约束能量最小化

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摘要

We study the ionization energy, electron affinity, and the π → π- (1La) excitation energy of the anthracene molecule, by means of variational quantum Monte Carlo (QMC) methods based on a Jastrow correlated antisymmetrized geminal power (JAGP) wave function, developed on molecular orbitals (MOs). The MO-based JAGP ansatz allows one to rigorously treat electron transitions, such as the HOMO → LUMO one, which underlies the 1La excited state. We present a QMC optimization scheme able to preserve the rank of the antisymmetrized geminal power matrix, thanks to a constrained minimization with projectors built upon symmetry selected MOs. We show that this approach leads to stable energy minimization and geometry relaxation of both ground and excited states, performed consistently within the correlated QMC framework. Geometry optimization of excited states is needed to make a reliable and direct comparison with experimental adiabatic excitation energies. This is particularly important in π-conjugated and polycyclic aromatic hydrocarbons, where there is a strong interplay between low-lying energy excitations and structural modifications, playing a functional role in many photochemical processes. Anthracene is an ideal benchmark to test these effects. Its geometry relaxation energies upon electron excitation are of up to 0.3 eV in the neutral 1La excited state, while they are of the order of 0.1 eV in electron addition and removal processes. Significant modifications of the ground state bond length alternation are revealed in the QMC excited state geometry optimizations. Our QMC study yields benchmark results for both geometries and energies, with values below chemical accuracy if compared to experiments, once zero point energy effects are taken into account. © 2015 AIP Publishing LLC.
机译:我们通过基于Jastrow相关反对称双晶幂(JAGP)波函数的变分量子蒙特卡洛(QMC)方法研究了蒽分子的电离能,电子亲和力和π→π-(1La)激发能,在分子轨道(MOs)上开发。基于MO的JAGP ansatz允许严格处理电子跃迁,例如HOMO→LUMO跃迁,它是1La激发态的基础。我们提出了一种QMC优化方案,该方案能够通过基于对称选定MO的投影仪的约束最小化来保留反对称双精度功率矩阵的等级。我们表明,这种方法导致在相关QMC框架内一致执行的稳定的能量最小化以及基态和激发态的几何弛豫。需要对激发态进行几何优化,以便与绝热激发能进行可靠,直接的比较。这在π共轭和多环芳烃中尤为重要,在低能激发和结构修饰之间存在很强的相互作用,在许多光化学过程中起着重要作用。蒽是测试这些效果的理想基准。在中性1La激发态下,其在电子激发时的几何弛豫能高达0.3 eV,而在电子添加和去除过程中,其弛豫能约为0.1 eV。 QMC激发态几何优化中揭示了基态键长交替的显着变化。我们的QMC研究得出了几何形状和能量的基准结果,如果将零点能量影响考虑在内,则与实验相比,该值低于化学精度。 ©2015 AIP Publishing LLC。

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