Vertical spectrum of the C2H2+ system. An open shell (SC)2-CAS-SDCI study

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Molecular Spectroscopy

Background:

• Acetylene (C2H2) is a fundamental molecule with complex electronic structure.
• Accurate theoretical prediction of its ionization potentials and excited states is crucial for understanding its chemical behavior.

Purpose of the Study:

• To calculate the main ionization potentials and excited states of acetylene using an advanced computational method.
• To analyze the composition and symmetries of these electronic states.

Main Methods:

• Application of the open-shell (SC)(2)-CAS-SDCI method with an atomic natural orbital (ANO) basis set.
• Generation of single and double excitations from a CAS space.
• Correction of the CI matrix using the (SC)(2) procedure to address size-extensivity errors.

Main Results:

• Accurate calculation of ionization potentials for outer- and inner-valence states with a mean absolute error of 0.1 eV.
• Detailed reporting and discussion of excited states across various symmetries (Sigma, Pi, Delta).
• Identification of significant mixing between 3sigma(g) (-1) and 2sigma(g) (-1) processes in specific excited states when using Rydberg functions.

Conclusions:

• The (SC)(2)-CAS-SDCI method provides accurate results for acetylene's electronic spectrum.
• The study highlights discrepancies with previous large MR-CI calculations, emphasizing the importance of the chosen computational approach.
• Significant electronic state mixing is observed, offering insights into acetylene's excited-state dynamics.