We introduce a new variant of the complete active space second-order perturbation theory (CASPT2) method that performs similarly to multistate CASPT2 (MS-CASPT2) in regions of the potential energy surface where the electronic states are energetically well separated and is akin to extended MS-CASPT2 (XMS-CASPT2) in case the underlying zeroth-order references are near-degenerate. Our approach follows a recipe analogous to that of XMS-CASPT2 to ensure approximate invariance under unitary transformations of the model states and a dynamic weighting scheme to smoothly interpolate the Fock operator between state-specific and state-average regimes. The resulting extended dynamically weighted CASPT2 (XDW-CASPT2) methodology possesses the most desirable features of both MS-CASPT2 and XMS-CASPT2, that is, the ability to provide accurate transition energies and correctly describe avoided crossings and conical intersections. The reliability of XDW-CASPT2 is assessed on a number of molecular systems. First, we consider the dissociation of lithium fluoride, highlighting the distinctive characteristics of the new approach. Second, the invariance of the theory is investigated by studying the conical intersection of the distorted allene molecule. Finally, the relative accuracy in the calculation of vertical excitation energies is benchmarked on a set of 26 organic compounds. We found that XDW-CASPT2, albeit being only approximately invariant, produces smooth potential energy surfaces around conical intersections and avoided crossings, performing equally well to the strictly invariant XMS-CASPT2 method. The accuracy of vertical transition energies is almost identical to MS-CASPT2, with a mean absolute deviation of 0.01-0.02 eV, in contrast to 0.12 eV for XMS-CASPT2.

中文翻译：

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扩展的动态加权CASPT2：两全其美。

我们介绍了一种完整的主动空间二阶摄动理论（CASPT2）方法的新变体，该方法在电子能被能量很好地分离并类似于扩展的势能表面区域中与多态CASPT2（MS-CASPT2）相似如果基础零阶引用接近简并，则MS-CASPT2（XMS-CASPT2）。我们的方法遵循与XMS-CASPT2类似的方法，以确保在模型状态的统一变换下具有近似不变性，并采用动态加权方案在特定状态和平均状态之间平滑地插值Fock算子。由此产生的扩展的动态加权CASPT2（XDW-CASPT2）方法具有MS-CASPT2和XMS-CASPT2的最理想的功能，即，提供准确的跃迁能量并正确描述避免的交叉和圆锥形交叉的能力。XDW-CASPT2的可靠性在许多分子系统上得到了评估。首先，我们考虑氟化锂的离解，突出了这种新方法的独特特征。其次，通过研究扭曲的异戊二烯分子的圆锥形交点来研究理论的不变性。最后，以一组26种有机化合物为基准，计算垂直激发能的相对精度。我们发现，尽管XDW-CASPT2只是近似不变的，但它在圆锥形交叉点周围产生了光滑的势能面并避免了交叉，与严格不变的XMS-CASPT2方法一样，其表现同样出色。