Ab initio results for the low‐lying 1B1u 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 x2, 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.
Size-inconsistency effects in molecular properties for states with valence-Rydberg mixing: The low-lying pi --> pi * states of ethylene and butadiene. Robert J. Cave, J. Chem. Phys. 92, 2450 (1990), DOI:10.1063/1.457987.