Theoretical study on the core-excited states of the allyl using multi-reference methods with core-valence separation...

Area of Science:

• Quantum Chemistry
• Computational Spectroscopy
• Theoretical Chemistry

Background:

• Accurate calculation of core-excited states is crucial for interpreting X-ray absorption spectra (XAS).
• Existing methods may face challenges in balancing accuracy and computational efficiency for complex systems.
• Core-valence separation (CVS) approximations are vital for studying core electronic excitations.

Purpose of the Study:

• To develop and validate efficient multi-state perturbation theories (CVS-MS-NEVPT2 and CVS-SDSPT2) for core-excited states.
• To assess the accuracy and computational performance of these new methods compared to CVS-icMRCISD.
• To apply these methods to investigate the core-excited states of the allyl system and interpret experimental XAS.

Main Methods:

• Development of multi-state n-electron valence second-order perturbation theory with core-valence separation (CVS-MS-NEVPT2).
• Development of static-dynamic-static multi-state multi-reference second-order perturbation theory with CVS (CVS-SDSPT2).
• Utilized internally contracted multi-reference configuration interaction with single and double excitations with CVS (CVS-icMRCISD) as a benchmark.
• Pilot application to core-excited states of the allyl system and allyl cation.

Main Results:

• CVS-MS-NEVPT2 and CVS-SDSPT2 demonstrate significantly improved computational efficiency over CVS-icMRCISD while maintaining comparable accuracy.
• CVS-icMRCISD accurately reproduced experimental XAS peaks for the allyl system and allyl cation, with an average deviation of 0.25 eV.
• Vibrational analysis suggests potential electronic-vibronic coupling contributing to the ambiguity of the α band assignment in allyl XAS.

Conclusions:

• The developed CVS-MS-NEVPT2 and CVS-SDSPT2 methods provide a computationally efficient and accurate approach for studying core-excited states.
• These methods are suitable for medium-sized molecules, offering a practical alternative to more computationally demanding methods.
• The study highlights the importance of considering electronic-vibronic coupling in spectral assignments for core-excited states.