  We report a new implementation of multireference algebraic diagrammatic
construction theory (MR-ADC) for simulations of electron attachment and
ionization in strongly correlated molecular systems (EA/IP-MR-ADC). Following
our recent work on IP-MR-ADC [J. Chem. Theory Comput. 2019, 15, 5908], we
present the first implementation of the second-order MR-ADC method for electron
attachment (EA-MR-ADC(2)), as well as two extended second-order approximations
(EA- and IP-MR-ADC(2)-X) that incorporate a partial treatment of third-order
electron correlation effects. Introducing a small approximation for the
second-order amplitudes of the effective Hamiltonian, our implementation of EA-
and IP-MR-ADC(2)-X has a low O(M^5) computational scaling with the basis set
size M. Additionally, we describe an efficient algorithm for solving the
first-order amplitude equations in MR-ADC and partially-contracted second-order
N-electron valence perturbation theory (NEVPT2) that completely avoids
computation of the four-particle reduced density matrices without introducing
any approximations or imaginary-time propagation. For a benchmark set of eight
small molecules, carbon dimer, and a twisted ethylene, we demonstrate that EA-
and IP-MR-ADC(2)-X achieve accuracy similar to that of strongly-contracted
NEVPT2, while having a lower computational scaling with the active space size
and providing efficient access to transition properties.
