The core-valence separation (CVS) scheme177 allows one to extend standard methods for excited and ionized states to the core-level states. 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. Currently, CVS is implemented within EOM-EE/SF/IP-CCSD for energies and transition properties (oscillator strengths, NTOs, Dyson orbitals, exciton descriptors, ECD moments). CVS-EOM-EE-CCSD can be used to model NEXAFS, while CVS-EOM-IP-CCSD can be used to model XPS and XES. These methods can also be used to compute transient absorption spectra, e.g., valence pump/X-ray probe experiments.1162 The calculation of non-linear spectra, such as RIXS, is also possible. L-edge spectra (XAS and XPS) can be described by using state-interaction approach in which spin–orbit coupling is evaluated using non-relativistic CVS-EOM-EE states1163. Auger spectra can be computed using CVS-EOM-EE combined with the explicit treatment of the continuum via Feshbach-Fano approach1038 (see Section 7.10.10).
In Q-Chem, a slightly different version of CVS-EOM-EE-CCSD than the original theory by Coriani and Koch224 is implemented: the reference coupled-cluster amplitudes do not include core electrons.1161 To distinguish this method from the original,224 we refer in what follows to the Q-Chem implementation as frozen-core-ground-state/core-valence-separated EOM (FC-CVS-EOM) approach.1161
In the FC-CVS-EOM approach the ground-state parameters (amplitudes and Lagrangian multipliers) are computed within the frozen-core approximation, whereas the core-excitation energies and transition strengths are obtained imposing that at least one index in the EOM excitation (and ionization) operators refer to a core occupied orbital. Likewise, CVS is enforced in the calculation of the response states in RIXS.810
To ensure the best convergence of EOM equations, the calculation is edge-specific with respect to the highest lying edges (or deepest lying core orbitals): the frozen-core and CVS spaces are selected for each edge such that the core orbitals we are addressing in the excited state calculations are explicitly frozen in the ground state calculation and specifically included in the EOM calculation. Examples 188.8.131.52 and 184.108.40.206 below illustrate this point.
Although the convergence of FC-CVS-EOM is much more robust that that of regular EOM-CCSD, sometimes calculations would collapse to low-lying artificial states. If this happens, rerun the calculation using CVS_EOM_SHIFT to specify an approximate onset of the edge.
To invoke the CVS approximation, use METHOD = CCSD and CVS_EE_STATES instead of EE_STATES to specify the desired target states (likewise, CVS_EE_SINGLETS and CVS_EE_TRIPLETS can be used in exactly the same way as in regular EOM calculations). For ionized states, use CVS_IP_STATES or CVS_IP_ALPHA/CVS_IP_BETA. Spin-flip version can be deployed by using CVS_SF_STATES (this may be needed for computing SOCs and L-edge spectra). Preconverging single amplitudes can be invoked by CVS_EOM_PRECONV_SINGLES. Transition properties and Dyson orbitals can be computed either within CVS manifold or between CVS and valence manifolds (see Section 7.10.28 for definition of Dyson orbitals). CVS-EOM-CCSD is only available with CCMAN2.
Note: Core electrons must be frozen in CVS-EOM calculations. The exact definition of the core depends on the edge, so using default values may be not appropriate.
Note: The default setting (N_FROZEN_CORE = FC) does not work correctly in QM/MM calculations. One should specify the number of frozen core orbitals explicitly.
RIXS is a coherent two-photon process involving core-level states.810 The calculations of RIXS cross-sections require solving response equations, in the same fashion as in calculations of 2PA cross-sections (see Section 220.127.116.11). Currently, only calculations of RIXS cross-sections between the CCSD reference and the EOM-CCSD target states are possible. Because of the resonant nature of RIXS, damped response theory is used to handle singularities in the resolvent. In addition, CVS is enforced on the response vectors to eliminate their coupling with the ionization continuum. To set up RIXS calculation, one needs to set METHOD = EOM-CCSD and to specify the number of excited states to be included by using EE_STATES (note that only 2PA bright states need to be included) and to activate CVS by using CVS_EE_STATES asking for zero states. RIXS calculations is deployed by activating CC_EOM_RIXS. The option for performing wave-function analysis (e.g., NTO analysis) of the complex RIXS one-particle transition density matrices is also available. Conceptual details can be found in Ref. 808. Finally, one can request to compute REXS (cross-section for the “elastic” peak) by CC_REF_PROP = 1 (by default, REXS is not calculated) This feature is only available within fc-CVS-EOM-EE-CCSD.
Note: Since the RIXS response solutions within EOM-EE-CCSD often do not converge, fc-CVS-EOM-EE-CCSD RIXS calculations with CC_EOM_RIXS = 1 are recommended for smooth convergence. All other options are experimental.
To specify pumping frequencies and damping factor , use $rixs section:
$rixs damped_epsilon 0.005 Damping factor in hartree. omega_1 2306503 500 10 0 Pumping frequencies: starting w (cm-1), increment (cm-1), number of frequency points, algo (always 0 for now) omega_2 2200000 600 20 0 Emitted frequencies: starting w (cm-1), increment (cm-1), number of frequency points, algo (always 0 for now) $end
Damping factor (DAMPED_EPSILON) is specified in atomic units (0.005 is a good choice). OMEGA_1 specifies the starting pumping frequency (in cm), the increment in frequency scan (in cm), number of frequency points, and the algorithm for response equation (use zero until further notice). OMEGA_2 should be specified only for generating 2D RIXS scans. Example 18.104.22.168 illustrates the setup of RIXS calculations.
Note: For better convergence of the response equations, increase CC_DIIS_SIZE (25 is recommended) and consider using a larger damping factor.
In example 22.214.171.124, the orbital on the oxygen atom is frozen in the CCSD calculation (N_FROZEN_CORE = FC). In the EOM calculation, the CVS approximation is invoked (CVS_EE_SINGLETS), so that the core-excitation energies are obtained as the lowest excitations. The calculation of the oscillator strengths and rotatory strengths is activated by selecting CC_TRANS_PROP = 1 and the libwfa analysis is invoked by STATE_ANALYSIS = TRUE (see Section 10.2.9). Note that rotatory strengths will be zero for non-chiral systems.
Example 126.96.36.199 illustrates CVS-EOM-EE-CCSD calculations in a two-edge molecule (CO). In the present implementation, the calculation should be done separately for each edge. The first job computes carbon-edge states. Since the carbon orbital is the highest in energy (among the core orbitals of the molecule), the input for the C-edge is similar to example 188.8.131.52. Both the oxygen’s and the carbon’s orbitals are frozen in the reference CCSD calculation. In the EOM part, the carbon core-excited states are automatically selected. In this case, using default frozen core settings (N_FROZEN_CORE = FC) is equivalent to specifying N_FROZEN_CORE = 2. In the second input, the oxygen edge is computed. As the core-orbitals of oxygen lie deeper, the frozen core and CVS selection specifically targets the oxygen edge by using a smaller core. The 1 orbital of the oxygen atom is selected by N_FROZEN_CORE = 1. If the molecule has other edges, the deepest lying core orbitals, up to and including those of the edge of interest, should be selected by an appropriate value of N_FROZEN_CORE.
Example 184.108.40.206 illustrates calculation of the XES spectrum of benzene. Examples 220.127.116.11 and 18.104.22.168 illustrate calculations of Dyson orbitals between core-excited and core-ionized states and between core-excited and valence-ionized states.
Example 22.214.171.124 illustrates the setup of RIXS calculations.
Calculations of L-edge spectra using state-interaction approach1163 entails a two-step procedure. First, Q-Chem computes necessary CVS-EOM states and SOCs. In the second step, the spin-orbit perturbed spectrum is computed by a post-processing script. Scripts, detailed instructions, and examples can be found here1164.
$molecule 0 1 O 0.0000 0.0000 0.1173 H 0.0000 0.7572 -0.4692 H 0.0000 -0.7572 -0.4692 $end $rem METHOD = eom-ccsd BASIS = aug-cc-pVDZ CVS_EE_SINGLETS = [3,0,2,1] N_FROZEN_CORE = fc CC_TRANS_PROP = true EOM_PRECONV_SINGLES = true STATE_ANALYSIS = true !invoke libwa to compute NTOs and exciton descriptors MOLDEN_FORMAT = true NTO_PAIRS = 3 POP_MULLIKEN = true $end
$comment CO, carbon edge $end $molecule 0 1 O 0.0000 0.0000 0.913973 C 0.0000 0.0000 -1.218243 $end $rem METHOD = eom-ccsd BASIS = aug-cc-pVDZ INPUT_BOHR = true CVS_EE_SINGLETS = [2,0,2,2] N_FROZEN_CORE = fc EOM_PRECONV_SINGLES = true CC_TRANS_PROP = true $end @@@ $comment CO, oxygen edge $end $molecule read $end $rem METHOD = eom-ccsd BASIS = aug-cc-pVDZ CVS_EE_SINGLETS = [2,0,2,2] N_FROZEN_CORE = 1 EOM_PRECONV_SINGLES = true CC_TRANS_PROP = true $end
$comment RI-MP2/cc-pVTZ optimized geometry. XES calculation. $end $molecule 0 1 H 2.4750347531 0.0000000000 0.0000000000 C 1.3935929418 0.0000000000 0.0000000000 C 0.6967964709 1.2068868901 0.0000000000 H 1.2375173766 2.1434429715 0.0000000000 C -0.6967964709 1.2068868901 0.0000000000 H -1.2375173766 2.1434429715 0.0000000000 C -1.3935929418 0.0000000000 0.0000000000 H -2.4750347531 0.0000000000 0.0000000000 C -0.6967964709 -1.2068868901 0.0000000000 H -1.2375173766 -2.1434429715 0.0000000000 C 0.6967964709 -1.2068868901 0.0000000000 H 1.2375173766 -2.1434429715 0.0000000000 $end $rem BASIS = 6-31G* METHOD = eom-ccsd IP_STATES = [3,2,1,1,0,1,2,2] !All valence Koompans-like ionized states except for 3 lowest ones CVS_IP_STATES = [2,1,0,0,0,0,1,2] !All core-ionized states CC_TRANS_PROP = 2 !Compute transitions between all pairs of EOM states CC_MEMORY = 8000 !8 GB $end
$comment CVS-IP/CVS-EE Dyson orbitals, formaldehyde $end $molecule 0 1 C H 1 1.096135 H 1 1.096135 2 116.191164 O 1 1.207459 2 121.904418 3 -180.000000 0 $end $rem METHOD = eom-ccsd BASIS = cc-pVDZ ! Please do not use BASIS2 SCF_CONVERGENCE = 8 CVS_IP_STATES = [1,0,0,0] CVS_EE_STATES = [1,0,1,0] CC_DO_DYSON = true CC_TRANS_PROP = 2 !Compute all EOM-to-EOM transitions $end
$comment IP/CVS-EE Dyson orbitals, formaldehyde $end $molecule 0 1 C H 1 1.096135 H 1 1.096135 2 116.191164 O 1 1.207459 2 121.904418 3 -180.000000 0 $end $rem METHOD = eom-ccsd BASIS = cc-pVDZ SCF_CONVERGENCE = 8 IP_STATES = [1,0,0,0] ! Valence a1 hole CVS_EE_STATES = [1,0,0,0] CC_DO_DYSON = true CC_TRANS_PROP = 2 !Compute all EOM-to-EOM transitions $end
$comment CVS-IP/CVS-EE Dyson orbitals, formaldehyde $end $molecule 0 1 C H 1 1.096135 H 1 1.096135 2 116.191164 O 1 1.207459 2 121.904418 3 -180.000000 0 $end $rem BASIS = cc-pVDZ SCF_CONVERGENCE = 8 METHOD = eom-ccsd IP_STATES = [1,0,0,0] Valence a1 hole CVS_EE_STATES = [1,0,0,0] CC_DO_DYSON = true CC_TRANS_PROP = true ! required to activate a Dyson orbitals job $end
$comment RI-MP2/cc-pVTZ optimized geometry. Pump XAS transition peak A at 285.97 eV, only one frequency point. $end $molecule 0 1 H 2.4750347531 0.0000000000 0.0000000000 C 1.3935929418 0.0000000000 0.0000000000 C 0.6967964709 1.2068868901 0.0000000000 H 1.2375173766 2.1434429715 0.0000000000 C -0.6967964709 1.2068868901 0.0000000000 H -1.2375173766 2.1434429715 0.0000000000 C -1.3935929418 0.0000000000 0.0000000000 H -2.4750347531 0.0000000000 0.0000000000 C -0.6967964709 -1.2068868901 0.0000000000 H -1.2375173766 -2.1434429715 0.0000000000 C 0.6967964709 -1.2068868901 0.0000000000 H 1.2375173766 -2.1434429715 0.0000000000 $end $rem BASIS = 6-31(+,+)G** METHOD = eom-ccsd CVS_EE_STATES = [0,0,0,0,0,0,0,0] just to invoke CVS EE_STATES = [10,10,10,10,0,0,0,0] 10 states in each 2PA active irrep CC_REF_PROP = 1 ! Calculate REXS in addition to RIXS CC_EOM_RIXS = 1 ! Activate RIXS calculation using fc-CVS-EOM-EE-CCSD CC_DIIS_SIZE = 25 ! Use for better convergence of response equations CC_MEMORY = 8000 !8 GB mem_total = 8500 $end $rixs omega_1 2306503 500 1 0 damped_epsilon 0.005 $end