Local Coupled Cluster response for open-shell systems

大分子电子激发态性质的计算在物理、化学和生物学等领域有着重要的应用。在以前的项目中，我们成功地实现了分析核能梯度的地面和激发单重态和三重态的封闭壳层分子在本地CC 2响应的水平。这允许激发态几何优化在该理论水平的系统的叶绿素-a的大小。此外，我们设计了一个层次的高效耦合团簇响应方法计算电离势（IPs. This项目的延续的目标是这种方法的扩展到开壳层系统。特别是，我们的目标是大自由基的激发态。这些物质通常在光化学和光催化过程中作为中间体发挥重要作用，可以通过光谱方法进行检测。在本项目中，我们打算从两个方面来实现这一目标，即，(i)传统的基于自旋轨道的开壳层耦合团簇响应理论，以及（ii）IPs的耦合响应理论。在后一种情况下，自由基的激发能计算为两个IP的差。参考波函数是相关的封闭壳层分子与N+1电子，和电离态代表自由基得到的纯二重态（没有任何自旋污染），我们打算实现激发能，以及跃迁强度，一阶属性，和分析的核梯度的基态和激发态的自由基。局部近似和密度拟合将被用来显着降低计算成本，并以这种方式，使方法适用于大型系统。我们认为，在本项目过程中开发的程序对研究光化学和光催化反应中的自由基中间体是非常有用的。

The calculation of properties of electronically excited states of large molecules is of great importance for many applications in physics, chemistry, and biology. In the previous project we successfully implemented analytical nuclear energy gradients for ground and excited singlet and triplet states of closed-shell molecules at the level of local CC2 response. This allows for excited state geometry optimizations at that level of theory for systems of the size of chlorophyllide-a. Furthermore, we devised a hierarchy of efficient coupled cluster response methods for calculating ionization potentials (IPs).The goal of this project continuation is the extension of this methodology to open shell systems. In particular we aim at excited states of large radicals. These often play an important role as intermediates in photochemical and photocatalytical processes and can be detected by means of spectroscopical methods. In the project we intend to approach this goal from two sides, i.e., (i) by traditional spin orbital based open-shell coupled cluster response theory, and (ii) by coupled response theory for IPs. In the latter case the excitation energies of the radical are computed as differences of two IPs. The reference wavefunction is that of the related closed-shell molecule with N+1 electrons, and the ionized states representing the radical are obtained as pure doublet states (without any spin contamination).We intend to implement excitation energies, as well as transition strengths, first-order properties, and analytical nuclear gradients for ground and excited states of radicals. Local approximations and density fitting will be utilized to dramatically reduce the computational cost and, in this way, to make the methods applicable to large systems. We think that the programs developed in the course of this project will be very useful to study radical intermediates in photochemical and photocatalytical reactions.