The core-valence separation (CVS) approximation^{155} 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/IP-CCSD for energies
and transition properties (oscillator strengths, NTOs, Dyson orbitals, exciton
descriptors). CVS-EOM-EE-CCSD can be used to model NEXAFS.

In Q-Chem, a slightly different version of CVS-EOM-EE-CCSD than the original
theory by Coriani and Koch^{190} is implemented: the reference
coupled-cluster amplitudes do not include core electrons^{944}. To
distinguish this method from the original^{190}, below we refer to
the Q-Chem implementation as frozen-core-ground-state/core-valence-separated
EOM (FC-CVS-EOM) approach.^{944}

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 strengths are obtained imposing that at least one index in the EOM excitation (and ionization) operators refer to a core occupied orbital.

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 7.8.5.2 and 7.8.5.2 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. Transition properties and Dyson orbitals can be computed either within CVS manifold or between CVS and valence manifolds (see Section 7.8.23 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.

CVS_EOM_SHIFT

Specifies energy shift in CVS-EOM calculations.

TYPE:

INTEGER

DEFAULT:

0

OPTIONS:

$n$
corresponds to $n\cdot {10}^{-3}$
hartree shift (i.e., 11000 = 11 hartree); solve for
eigenstates around this value.

RECOMMENDATION:

Improves the stability of the calculations.

Similar to ground-state CCSD calculations described in Section 6.15,
single precision can be used in EOM-CC and EOM-MP2 calculations ^{732}.
Currently, the following variants of EOM are supported: EE, SF,IP, EA;
both in standard and RI/CD implementations, for energies and properties evaluation.
If you wish to use single-precision version of EOM, please first read Section 6.15 for
basic setup of single-precision coupled-cluster calculation.
Here we describe only additional EOM-specific keywords.

Precision selection is controlled by the
EOM_SINGLE_PREC keyword:
$0$ corresponds to double-precision calculation and $1$ corresponds to single-precision
calculation.
EOM-specific convergence criteria are controlled by the same keywords as in the
double precision, but the same rule as for CCSD applies: too tight thresholds may
cause issues with convergence. The default Davidson threshold ${10}^{-5}$ works well for
most cases ^{732}.

The keyword CC_SP_DM controlls calculation of intermediates, density matrices, and ${S}^{2}$ for EOM calculations in the same manner as for CCSD, which is described in Section 6.15.

Calculations of analytical gradients require solving amplitude-response equations, which can be done on single precision as well; this is activated by EOM_ARESP_SINGLE_PREC=1.

EOM_SINGLE_PREC

Precision selection for EOM-CC/MP2 calculations. Available in CCMAN2 only.

TYPE:

INTEGER

DEFAULT:

0
double-precision calculation

OPTIONS:

1
single-precision calculation
2
single-precision calculation is followed by double-precision clean-up iterations

RECOMMENDATION:

Do not set too tight convergence criteria when use single precision

EOM_ARESP_SINGLE_PREC

Precision selection for amplitude response EOM equations. Available in CCMAN2 only.

TYPE:

INTEGER

DEFAULT:

0
double-precision calculation

OPTIONS:

1
single-precision calculation

RECOMMENDATION:

NONE

$comment Formaldehyde anion, single-precision calculation $end $molecule 0 1 C H 1 1.127888 H 1 1.127888 2 100.546614 $end $rem basis = cc-pvdz method = ccsd cholesky_tol = 3 EA_STATES = [1,0,0,0] cc_ref_prop = 1 Compute properties of the CCSD reference !SP keywords cc_single_prec = 1 cc_sp_t_conv = 4 cc_sp_e_conv = 6 cc_erase_dp_integrals = 0 ! set 1 to save disk space cc_sp_dm = 1 !EOM-specific keyword eom_single_prec = 1 $end

$molecule 0 1 N .034130 -.986909 0.000000 N -1.173397 .981920 0.000000 C -1.218805 -.408164 0.000000 C -.007302 1.702153 0.000000 C 1.196200 1.107045 0.000000 C 1.289085 -.345905 0.000000 O 2.310232 -.996874 0.000000 O -2.257041 -1.026495 0.000000 H .049329 -1.997961 0.000000 H -2.070598 1.437050 0.000000 H -.125651 2.776484 0.000000 H 2.111671 1.674079 0.000000 O 1.747914 -1.338382 -3.040233 H 2.180817 -1.817552 -2.333676 H 0.813180 -1.472188 -2.883392 $end $rem basis = cc-pvdz job_type = opt method = ccsd cc_state_to_opt = [1,1] mem_total = 30000 EE_TRIPLETS = [1] cc_sp_t_conv = 4 cc_sp_e_conv = 6 cc_single_prec = 1 eom_single_prec = 1 CC_SP_DM = 1 CC_EOM_PROP = 1 EOM_ARESP_SINGLE_PREC = 1 $end

In example 7.8.5.2, the $1s$ 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 is activated by selecting CC_TRANS_PROP = 1 and the libwfa analysis is invoked by STATE_ANALYSIS = TRUE (see Section 11.2.6).

Example 7.8.5.2 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 $1s$ orbital is the highest in energy (among the core $1s$ orbitals of the molecule), the input for the C-edge is similar to example 7.8.5.2. Both the oxygen’s and the carbon’s $1s$ 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$s$ 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.

Examples 7.8.5.2 and 7.8.5.2 illustrate calculations of Dyson orbitals between core-excited and core-ionized states and between core-excited and valence-ionized states.

$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 cvs_ee_singlets = [3,0,2,1] basis = aug-cc-pVDZ n_frozen_core = fc CC_TRANS_PROP = true eom_preconv_singles = true state_analysis = true !invoke libwa to compute NTOs and exciton descriptors ! libwa controls below 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 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 = true ! required to activate a Dyson orbitals job $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 = true ! required to activate a Dyson orbitals job $end

$comment CVS-IP/CVS-EE Dyson orbitals, formaldehyde $end $rem BASIS = cc-pVDZ JOB_TYPE = SP 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