Exact and approximate adiabatic connection formulae for the correlation energy in multireference ground and excited states

Recently it has been shown how to employ the adiabatic connection (AC) formalism to obtain correlation energy for multireference wavefunctions [K. Pernal, Phys. Rev. Lett. 120, 013001 (2018)]. Approximations to the exactAC formulation have been based on assuming that a one-electron reduced density matrix is constant along theAC path and by employing the extended random phase approximation. In this paper, the importance of these approximations is examined by comparing approximateAC integrands with their exact counterparts obtained for the hydrogen molecule in its ground and excited states. Encouraging results obtained for H-2 indicate thatACis a viable and promising approach to a correlation energy problem not only for ground but also for excited states of electronic systems. Published by AIP Publishing.