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Simulating X‐ray absorption spectra with complete active space self‐consistent field linear response methods
International Journal of Quantum Chemistry ( IF 2.2 ) Pub Date : 2020-11-27 , DOI: 10.1002/qua.26559
Benjamin Helmich‐Paris 1
Affiliation  

In this work, two approaches for simulating X‐ray absorption (XA) spectra with the complete active space self‐consistent field (CASSCF) linear response (LR) method are introduced. The first approach employs the well‐known core‐valence separation (CVS) approximation, which is predominantly used by many other electronic structure methods for simulating X‐ray spectra. The second ansatz uses the harmonic Davidson algorithm for finding interior eigenvalues that lie close to a target excitation energy shift and virtually solves a shifted‐and‐inverted (S&I) generalized eigenvalue problem. LR‐CASSCF K‐edge transition energies are systematically blueshifted though have consistently smaller errors than those of the CAS or restricted active space (RAS) configuration interaction (CI) methods. For simple molecules at which the core hole can only be created at a single site, the state‐specific RASSCF or n‐electron valence second‐order perturbation theory/RASCI gave more accurate principal K‐edge excitation energies. If the core hole can be created at multiple sites, the LR‐CASSCF approaches perform much better than RASSCF. Moreover, we observed that the LR‐CASSCF variants were the only MR methods discussed here that predicted correctly the order of O K‐edge features in the ozone molecule and the permanganate ion. The peak separation of edge features in ozone was as accurate as with equation‐of‐motion coupled cluster singles and doubles. The error of the CVS approximation turned out to be very system dependent and in some cases amounted up to 1.0 eV for the K‐edge excitation energies. Those CVS errors are still acceptable if one considers the observed deviation from the experimental reference by 5–11 eV. The deviations made in the XAS intensities were even more pronounced. CVS and the full S&I oscillator strengths could differ even by a factor of 2.8. Since the S&I approach is at least as efficient as the LR‐CASSCF method for valence excitations, future endeavors to improve the accuracy by accounting for dynamic correlation could be built on top of the full S&I approach.

中文翻译:

使用完整的有源空间自洽场线性响应方法模拟X射线吸收光谱

在这项工作中,介绍了两种使用完整的有源空间自洽场(CASSCF)线性响应(LR)方法模拟X射线吸收(XA)光谱的方法。第一种方法采用了众所周知的核价分离(CVS)近似方法,该方法主要被许多其他电子结构方法用来模拟X射线光谱。第二个ansatz使用谐波Davidson算法查找与目标激发能位移接近的内部特征值,并从根本上解决了位移和倒向(S&I)广义特征值问题。LR‐CASSCF的K边跃迁能量被系统地蓝移,尽管其误差始终小于CAS或受限活动空间(RAS)配置相互作用(CI)方法的误差。对于只能在单个位点产生中心孔的简单分子,国家特定的RASSCF或n电子价二阶微扰理论/ RASCI给出了更准确的主K边激发能。如果可以在多个位置创建岩心孔,则LR-CASSCF方法的性能要比RASSCF好得多。此外,我们观察到,LR-CASSCF变体是这里讨论的唯一能够正确预测臭氧分子和高锰酸根离子中OK边缘特征顺序的MR方法。臭氧中边缘特征的峰分离与运动方程耦合的单簇和双簇一样准确。事实证明,CVS近似误差与系统密切相关,在某些情况下,对于K边激励能量,误差高达1.0 eV。如果人们认为观测到的偏离实验基准的偏差为5-11 eV,则这些CVS误差仍然可以接受。XAS强度的偏差甚至更加明显。CVS和整个S&I振荡器的强度可能相差2.8倍。由于S&I方法至少对价激发具有与LR-CASSCF方法相同的效率,因此可以在完整的S&I方法基础上建立将来通过考虑动态相关性来提高准确性的工作。
更新日期:2020-12-23
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