In experiments using high-energy radiation (such as X-ray spectroscopy, EXAFS,
NEXAFS, XAS) core electrons can be ionized or excited to low-lying virtual
orbitals. There are several ways to compute ionization or excitation energies
of core electrons in Q-Chem. Standard approaches for excited and ionized
states need to be modified to tackle core-level states, because these states
have very high energies and are embedded in the ionization continuum (*i.e.*,
they are Feschbach resonances^{800}).

The most robust and accurate strategy is to invoke many-body methods, such as
EOM or ADC, together with the core-valence separation (CVS)
approximation^{155}. In this approach, the excitations
involving core electrons are decoupled from the rest of the configurational
space. This allows one to reduce computational costs and decouple the highly
excited core states from the continuum. These methods are described in
Sections 7.8.5 and 7.9.4.

Within EOM-CC formalism, one can also use an approximate EOM-EE/IP methods in
which the target states are described by single excitations and double
excitations are treated perturbatively; these methods are described in
Section 7.8.10. While being moderately useful, these methods are
less accurate than the CVS-EOM variants^{800}.

In addition, one can use the $\mathrm{\Delta}E$ approach, which amounts to a simple energy difference calculation in which core ionization is computed from energy differences computed for the neutral and core-ionized state. It is illustrated by example 7.11 below.

$molecule 0,1 C 0.000000 0.000000 0.000000 H 0.631339 0.631339 0.631339 H -0.631339 -0.631339 0.631339 H -0.631339 0.631339 -0.631339 H 0.631339 -0.631339 -0.631339 $end $rem EXCHANGE = HF CORRELATION = CCSD BASIS = 6-31G* MAX_CIS_CYCLES = 100 $end @@@ $molecule +1,2 C 0.000000 0.000000 0.000000 H 0.631339 0.631339 0.631339 H -0.631339 -0.631339 0.631339 H -0.631339 0.631339 -0.631339 H 0.631339 -0.631339 -0.631339 $end $rem UNRESTRICTED = TRUE EXCHANGE = HF BASIS = 6-31G* MAX_CIS_CYCLES = 100 SCF_GUESS = read Read MOs from previous job and use occupied as specified below CORRELATION = CCSD MOM_START = 1 Do not reorder orbitals in SCF procedure! $end $occupied 1 2 3 4 5 2 3 4 5 $end

In this job, we first compute the HF and CCSD energies of neutral CH${}_{4}$:
${E}_{\mathrm{SCF}}=-40.1949062375$ and ${E}_{\mathrm{CCSD}}=-40.35748087$ (HF orbital energy
of the neutral gives the Koopmans IE, which is 11.210 hartree = 305.03 eV). In the
second job, we do the same for core-ionized CH${}_{4}$. To obtain the desired SCF
solution, MOM_START option and *$occupied* keyword are used. The
resulting energies are ${E}_{\mathrm{SCF}}=-29.4656758483$ ($\u27e8{S}^{2}\u27e9$ =
0.7730) and ${E}_{\mathrm{CCSD}}=-29.64793957$. Thus, $\mathrm{\Delta}{E}_{\mathrm{CCSD}}=(40.357481-29.647940)=10.709$ hartree = 291.42 eV.

This approach can be further extended to obtain multiple excited states involving core electrons by performing CIS, TDDFT, or EOM-EE calculations.

Note: This approach often leads to convergence problems in correlated calculations.

Finally, one can also use the following trick illustrated by example 7.11.

$molecule 0,1 C 0.000000 0.000000 0.000000 H 0.631339 0.631339 0.631339 H -0.631339 -0.631339 0.631339 H -0.631339 0.631339 -0.631339 H 0.631339 -0.631339 -0.631339 $end $rem EXCHANGE = HF BASIS = 6-31G* MAX_CIS_CYCLES = 100 CORRELATION = CCSD CCMAN2 = false N_FROZEN_CORE = 4 Freeze all valence orbitals IP_STATES = [1,0,0,0] Find one EOM_IP state $end $reorder_mo 5 2 3 4 1 5 2 3 4 1 $end

Here we use EOM-IP to compute core-ionized states. Since core states are very high in energy, we use “frozen core” trick to eliminate valence ionized states from the calculation. That is, we reorder MOs such that our core is the last occupied orbital and then freeze all the rest. The so computed EOM-IP energy is 245.57 eV. From the EOM-IP amplitude, we note that this state of a Koopmans character (dominated by single core ionization); thus, canonical HF MOs provide good representation of the correlated Dyson orbital. The same strategy can be used to compute core-excited states.

Note: The accuracy of this approach is rather poor and is similar to the Koopmans approximation.