  We explore several random phase approximation (RPA) correlation energy
variants within the adiabatic-connection fluctuation-dissipation theorem
approach. These variants differ in the way the exchange interactions are
treated. One of these variants, named dRPA-II, is original to this work and
closely resembles the second-order screened exchange (SOSEX) method. We discuss
and clarify the connections among different RPA formulations. We derive the
spin-adapted forms of all the variants for closed-shell systems, and test them
on a few atomic and molecular systems with and without range separation of the
electron-electron interaction.
