Condon近似下多原子分子吸收光谱的从头算半经典计算
On-the-fly ab initio semiclassical evaluation of absorption spectra of polyatomic molecules beyond the Condon approximation
摘要：
为了评估超越Condon近似的电子振动光谱，我们通过考虑由于电子跃迁偶极矩对核坐标的依赖性而引起的Herzberg-Teller贡献来扩展在飞从头算解冻高斯近似。扩展的解冻高斯近似进行了测试的苯基自由基和苯的电子吸收光谱：计算的光谱再现实验数据，比标准的全球谐波方法更准确，证实了非谐性的意义。此外，扩展的方法提供了一个工具来量化的赫兹伯格-泰勒贡献：我们表明，在苯基自由基，非谐性超过赫兹伯格-泰勒贡献，而在苯中，赫兹伯格-泰勒贡献是必不可少的，因为过渡是电子禁戒和康登近似产生零光谱。令人惊讶的是，这两个绝热谐波谱优于垂直谐波模型，它更好地描述了Franck-Condon区域。最后，我们提供了一个简单的配方取向平均光谱，有效超越康登近似，和过渡偶极子，其梯度，和非绝热耦合矢量之间的关系。
英文摘要：
To evaluate vibronic spectra beyond the Condon approximation, we extend the on-the-fly ab initio thawed Gaussian approximation by considering the Herzberg-Teller contribution due to the dependence of the electronic transition dipole moment on nuclear coordinates. The extended thawed Gaussian approximation is tested on electronic absorption spectra of phenyl radical and benzene: Calculated spectra reproduce experimental data and are much more accurate than standard global harmonic approaches, confirming the significance of anharmonicity. Moreover, the extended method provides a tool to quantify the Herzberg-Teller contribution: we show that in phenyl radical, anharmonicity outweighs the Herzberg-Teller contribution, whereas in benzene, the Herzberg-Teller contribution is essential, since the transition is electronically forbidden and Condon approximation yields a zero spectrum. Surprisingly, both adiabatic harmonic spectra outperform those of the vertical harmonic model, which describes the Franck-Condon region better. Finally, we provide a simple recipe for orientationally averaging spectra, valid beyond Condon approximation, and a relation among the transition dipole, its gradient, and nonadiabatic coupling vectors.
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