Nonlinear Spectroscopy and Magnetic Circulardichroism with the Approximate Coupled-Cluster Singles-and-Doubles Method RI-CC2

在第二个资助期内，在第一个资助期内发展起来的RI-CC2的微扰诱导跃迁矩的实施应向两个方向扩展：首先是磁场诱导的跃迁矩，用于描述磁圆二色性（MCD），其次是两个激发态之间的跃迁，包括单重态和三重态之间的自旋轨道耦合。MCD光谱在实验中经常被用于获得补充紫外/可见吸收光谱结果的附加信息，因为吸收光谱中重叠的波段在MCD光谱中可能具有相反的符号。对于闭壳有机分子，MCD谱由所谓的B项主导，它描述了由磁场引起的振荡器强度的变化。尽管MCD光谱在实验中具有重要意义，但有机分子的MCD光谱的理论计算目前仅限于密度泛函理论，该理论在电荷转移和Rydberg激发方面存在已知缺陷，或者在相当小的分子的相关波函数水平上存在缺陷。利用相同分辨率近似（RI-CC2）对近似耦合簇单双模型CC2的MCD谱进行有效实现，将极大地扩展相关波函数水平上MCD理论研究的可能性。

In a second funding period the implementation of perturbation-induced transition moments for RI-CC2, developed in the first period, should be extended in two directions:firstly towards transition moments induced by magnetic fields, to describe magnetic circular dichroism (MCD), and secondly towards transitions between two excited states including spin-orbit couplingfor transitions between singlet and triplet states.MCD spectroscopy is experimentally often applied to obtain additional information which complements the results form UV/Vis absorption spectra since bands which overlap in the absorption spectrum can have opposite signs in the MCD spectrum. For closed-shell organic molecules the MCD spectrum is dominated by the so-called B term which describes a change in the oscillator strengths induced by the magnetic field. Despite the experimental importance of MCD spectroscopy theoretical computations of the MCD spectra of organic molecules are currently limited to density functional theory, which has knowndefiencies for charge-transfer and Rydberg excitations, or at correlated wavefunction levels to rather small molecules.An efficient implementation of MCD spectra for the approximate coupled-cluster singles-and-doubles model CC2 with the resolution-of-the-identity approximation (RI-CC2) would greatly extend the possibilities for theoretical studies of MCD at a correlated wavefunction level.