Multireference coupled-cluster method: Ionization potentials and excitation energies for ketene and diazomethane

A linked, size-extensive multireference coupled-cluster method (MRCC) is presented, and has been used to calculate the ionization potentials and excitation energies of the interesting molecules ketene and diazomethane. Comparisons are made with experimental numbers and theoretical results from configuration-interaction (CI) studies. Good agreement is found between the MRCC results, CI calculations, and experiment.