Quantum dynamics simulations, with the Multiconfigurational Time Dependent Hartree method, are used to assign the very broad absorption spectrum of Cytosine in gas phase and study the relation between spectral features and the ultrafast internal conversions among its excited states. For each of the four populated tautomers of Cytosine we built a Linear Vibronic Coupling model, comprising all the low energy excited states up to ∼6.5 eV (7–9 states). We used Density Functional Theory and a general diabatization scheme based on the projection of the excited states on a set of reference ones. Vibronic progressions and inter-state couplings dominate the spectral shape, which is in nice agreement with experiment. Inter-state couplings contribute to the loss of vibronic resolution and to the spread of the absorption intensity along a large energy range. Their importance is different for each tautomer and is connected to the different decay times of the bright states.

利用多构型时变哈特里（Hartree）方法进行量子动力学模拟，以分配胞嘧啶在气相中的非常宽的吸收光谱，并研究光谱特征与其激发态之间的超快内部转化之间的关系。对于胞嘧啶的四个组装互变异构体中的每一个，我们建立了线性振动耦合模型，该模型包括所有低能量激发态，最高约6.5 eV（7–9个状态）。我们使用密度泛函理论和基于激发态在一组参考态上的投影的通用绝热方案。振动曲线和状态间耦合占主导地位的光谱形状，这与实验很好地吻合。状态间耦合会导致电子振动分辨率的下降，并导致吸收强度沿较大的能量范围扩展。