Coordinate rotation studies of H−, He−, Be−, Mg− resonances: Basis set and configuration list dependence

We have performed coordinate rotated configuration interaction calculations on well‐studied Feshbach resonances of H− and He− and on 2P shape resonances of Be− and Mg−. The focus of our efforts was the dependence of computed resonance energies on both the quality of the atomicorbital basis and the level of treatment of electron correlation. Our results indicate that great care must be taken to guarantee that a basis is adequate; commonly used quantum‐chemistry bases are probably far from satisfactory. Our findings also indicate that a proper treatment of inner‐shell orbitals within coordinate rotation calculations is a formidable task. We are therefore encouraged to look carefully for modified coordinate rotation techniques that focus on the active valence‐level orbitals and may avoid spurious complex energies arising from improper treatment of inner shells.