Size‐Inconsistency Effects in Molecular Properties for States with Valence‐Rydberg Mixing: The Low‐Lying π→π∗ States of Ethylene and Butadiene

Size-inconsistency effects in molecular properties for states with valence-Rydberg mixing: The low-lying pi-> pi * states of ethylene and butadiene. Ab initio results for the low-lying I B I u states of ethylene are used to discuss size-inconsistency effects on calculated molecular properties for states where valence-Rydberg mixing is important. Results for the expectation value of x 2 , x being the coordinate perpendicular to the plane of the molecule, are presented from quasidegenerate variational perturbation theory and multireference configuration-interaction calculations. These results are compared with values from previous studies. It is argued that size inconsistency in configuration-interaction studies can have a significant effect on estimated molecular properties. Calculations on several low-lying states of butadiene are also reported where similar size-inconsistency effects are found.