Butadiene is the simplest polyene and has long served as a model system for many chemical and spectroscopic properties. However, this small molecule has presented significant challenges to theoretical chemistry. The 2¹A_g state, which is dark but photochemically important, is a prime source of this difficulty. Previous studies attributed the notorious difficulty in treating this state to strong double excitation character of the 2¹A_g state, which prevents the application of linear response (LR) methods. Therefore, one would require methods with much higher computational cost, especially for the analogues of this state in longer polyenes, and consequently studies of longer polyenes are very limited. In the present work, we argue that the difficulty stems more significantly from the inherently multiconfigurational character of the ground state. In addition, we validate the possibility of employing LR time-dependent density functional theory to investigate such a state with reasonable accuracy.

中文翻译：

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双重在有争议2激发字符或静态相关性的参考状态的¹甲_克的国家反式丁二烯？

丁二烯是最简单的多烯，长期以来一直充当许多化学和光谱性质的模型系统。但是，这种小分子对理论化学提出了重大挑战。黑暗但在光化学上很重要的2 ¹ A _g状态是造成这种困难的主要原因。先前的研究将治疗这种状态的臭名昭著的困难归因于2 ¹ A _g的强双重激发特性状态，从而阻止了线性响应（LR）方法的应用。因此，人们将需要具有更高计算成本的方法，特别是对于较长多烯中该状态的类似物，因此，对较长多烯的研究非常有限。在当前的工作中，我们认为困难更多地源于基态固有的多构型特征。此外，我们验证了采用LR时变密度泛函理论以合理的精度研究这种状态的可能性。