Use of noncanonical orbitals in many-body perturbation theory

A formalism for dealing with noncanonical Hartree–Fock spin orbitals is presented within the framework of many-body perturbation theory. A test calculation on the planar methyl radical is carried out which displays certain features of open-shell calculations and which demonstrates the viability of using noncanonical spin orbitals. Results are found to be in good agreement with a previous configuration interaction calculation.