7.10.1 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. ¹⁴⁹⁴ Wormit M. et al.
Mol. Phys.
(2014), 112, pp. 774. Link ^{, 528} Harbach P. H., Wormit M., Dreuw A.
J. Chem. Phys.
(2014), 141, pp. 064113. Link 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. ⁷¹⁷ Krauter C. M., Pernpointner M., Dreuw A.
J. Chem. Phys.
(2013), 138, pp. 044107. Link ^{, 1494} Wormit M. et al.
Mol. Phys.
(2014), 112, pp. 774. Link 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. ¹⁴⁹⁴ Wormit M. et al.
Mol. Phys.
(2014), 112, pp. 774. Link ^{, 1451} Wenzel J., Wormit M., Dreuw A.
J. Comput. Chem.
(2014), 35, pp. 1900. Link ^{, 1452} Wenzel J., Wormit M., Dreuw A.
J. Chem. Theory Comput.
(2014), 10, pp. 4583. Link ^{, 1450} Wenzel J. et al.
J. Chem. Phys.
(2015), 142, pp. 214104. Link 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. ³¹⁷ Dempwolff A. L. et al.
J. Chem. Phys.
(2019), 150, pp. 064108. Link ^{, 315} Dempwolff A. L. et al.
J. Chem. Phys.
(2020), 152, pp. 024113. Link ^{, 316} Dempwolff A. L. et al.
J. Chem. Phys.
(2020), 152, pp. 024125. Link ^{, 314} Dempwolff A. L. et al.
J. Chem. Phys.
(2021), 154, pp. 104117. Link In addition to Møller-Pesset based (MP-)ADC, regular schemes based on a size-consistent Brillouin-Wigner (BWs) partitioning are available for excited states (see also 6.8). Currently, BWs-ADC(0), BWs-ADC(2)-s, BWs-ADC(2)-x and BWs-ADC(3) are implemented in combination with the resolution-of-identity approximation.