Coupled Nuclearand Electron Dynamics in ChlorophyllUnraveled by XMS-CASPT2 X‑ray Absorption Spectra

Attosecond spectroscopy, especially time-resolved X-ray absorption spectra (XAS), enables direct observation of ultrafast molecular dynamics. The complementary and even preceding development of theoretical simulations can offer the necessary guidance and stimulate new experiments. In this work, we simulated high-level XAS for the magnesium and nitrogen K-edge of chlorophyll <i>a</i>. In our previous work on the ultrafast relaxation process in the Q-band, our quantum dynamics simulations found the <i>Q</i>_<i>x</i> and <i>Q</i>_<i>y</i> states to be energetically close and therefore strongly coupled. Here, we analyze the strong coupling between <i>Q</i>_<i>x</i> and <i>Q</i>_<i>y</i> via XAS, indicating promising possibilities for experimental observation. The excited-state energies, potential energy surfaces, and XAS are computed at the XMS-CASPT2 level of theory to capture the complex multireference character of chlorophyll excitations. In our simulated spectra, we could follow the ultrafast population transfer between <i>Q</i>_<i>x</i> and <i>Q</i>_<i>y</i> and thus draw conclusions about the strong vibrational coupling between them.