7.11.1 Introduction

The ADC($n$) family of correlated excited state methods is a series of size-extensive excited state methods based on perturbation theory. Each order $n$ of ADC presents the excited state equivalent to the well-known $n$th order Møller-Plesset perturbation theory for the ground state. Currently, the ADC variants ADC(0), ADC(1), ADC(2)-s, ADC(2)-x and ADC(3) are implemented in Q-Chem.^{1235, 425} The resolution-of-the-identity approximation can be used with any ADC variant. Additionally, there are spin-opposite scaling versions of both ADC(2) variants available.^{592, 1235} Core-excited states for the simulation of X-ray absorption spectra can be computed exploiting the core-valence separation (CVS) approximation. Currently, the CVS-ADC(1), CVS-ADC(2)-s, CVS-ADC(2)-x and CVS-ADC(3) methods are available.^{1235, 1198, 1199, 1197} Ionized and electron-attached states can be computed using the non-Dyson IP- and EA-ADC methods. Currently, the IP-ADC(2), IP-ADC(3), EA-ADC(2) and EA-ADC(3) methods are implemented.^{263, 261, 262, 260}