Correlation Energy Expressions from the Adiabatic-Connection Fluctuation-Dissipation Theorem Approach

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.