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(May 16, 2021)

It is important to ensure there are sufficient resources available for the
necessary integral calculations and transformations. For CCMAN/CCMAN2
algorithms, these resources are controlled using the *$rem* variables
CC_MEMORY, MEM_STATIC and MEM_TOTAL (see
Section 6.16).

The exact flavor of correlation treatment within equation-of-motion methods is
defined by METHOD (see Section 7.1). For
EOM-CCSD, once should set METHOD to EOM-CCSD, for EOM-MP2,
METHOD = EOM-CCSD, *etc.*. In addition, a specification of
the number of target states is required through XX_STATES (XX
designates the type of the target states, *e.g.*, EE, SF, IP, EA, DIP, DSF,
*etc.*). Users must be aware of the point group symmetry of the system being
studied and also the symmetry of the initial and target states of interest, as
well as symmetry of transition. It is possible to turn off the use of symmetry
by CC_SYMMETRY. If set to FALSE the molecule will be treated
as having ${C}_{1}$ symmetry and all states will be of $A$ symmetry.

Note: 1. In finite-difference calculations, the symmetry is turned off automatically, and the user must ensure that XX_STATES is adjusted accordingly. 2. In CCMAN, mixing different EOM models in a single calculation is only allowed in Dyson orbitals calculations. In CCMAN2, different types of target states can be requested in a single calculation.

Below we describe alternative way to specify correlation treatment in EOM-CC/CI
calculations. These keywords will be eventually phased out. By default, the
level of correlation of the EOM part of the wave function (*i.e.*, maximum
excitation level in the EOM operators $R$) is set to match
CORRELATION, however, one can mix different correlation levels for the
reference and EOM states by using EOM_CORR. To request a CI
calculation, set CORRELATION = CI and select type of CI expansion by
EOM_CORR. The table below shows default and allowed
CORRELATION and EOM_CORR combinations.

CORRELATION | Default | Allowed | Target states | CCMAN / |
---|---|---|---|---|

EOM_CORR | EOM_CORR | CCMAN2 | ||

CI | none | CIS, CIS(D) | EE, SF | y/n |

CISD | EE, SF, IP | y/n | ||

SDT, DT | EE, SF, DSF | y/n | ||

CIS(D) | CIS(D) | N/A | EE, SF | y/n |

CCSD, OD | CISD | EE, SF, IP, EA, DIP | y/y | |

SD(fT) | EE, IP, EA | n/y | ||

SD(dT), SD(fT) | EE, SF, fake IP/EA | y/n | ||

SD(dT), SD(fT), SD(sT) | IP | y/n | ||

SDT, DT | EE, SF, IP, EA, DIP, DSF | y/n |

Table 7.2 shows the correct combinations of CORRELATION and EOM_CORR for standard EOM and CI models.

Method | CORRELATION | EOM_CORR | Target states selection |

CIS | CI | CIS | EE_STATES |

EE_SINGLETS, EE_TRIPLETS | |||

SF-CIS | CI | CIS | SF_STATES |

CIS(D) | CI | CIS(D) | EE_STATES |

EE_SINGLETS, EE_TRIPLETS | |||

SF-CIS(D) | CI | CIS(D) | SF_STATES |

CISD | CI | CISD | EE_STATES |

EE_SINGLETS, EE_TRIPLETS | |||

SF-CISD | CI | CISD | SF_STATES |

IP-CISD | CI | CISD | IP_STATES |

CISDT | CI | SDT | EE_STATES |

EE_SINGLETS, EE_TRIPLETS | |||

SF-CISDT | CI | SDT or DT | SF_STATES |

EOM-EE-CCSD | CCSD | EE_STATES | |

EE_SINGLETS, EE_TRIPLETS | |||

EOM-SF-CCSD | CCSD | SF_STATES | |

EOM-IP-CCSD | CCSD | IP_STATES | |

EOM-EA-CCSD | CCSD | EA_STATES | |

EOM-DEA-CCSD | CCSD | DIP_STATES | |

DEA_SINGLETS, DEA_TRIPLETS | |||

EOM-DIP-CCSD | CCSD | DIP_STATES | |

DIP_SINGLETS, DIP_TRIPLETS | |||

EOM-2SF-CCSD | CCSD | SDT or DT | DSF_STATES |

EOM-EE-(2,3) | CCSD | SDT | EE_STATES |

EE_SINGLETS, EE_TRIPLETS | |||

EOM-SF-(2,3) | CCSD | SDT | SF_STATES |

EOM-IP-(2,3) | CCSD | SDT | IP_STATES |

EOM-SF-CCSD(dT) | CCSD | SD(dT) | SF_STATES |

EOM-SF-CCSD(fT) | CCSD | SD(fT) | SF_STATES |

EOM-IP-CCSD(dT) | CCSD | SD(dT) | IP_STATES |

EOM-IP-CCSD(fT) | CCSD | SD(fT) | IP_STATES |

EOM-IP-CCSD(sT) | CCSD | SD(sT) | IP_STATES |

The most relevant EOM-CC input options follow.

EE_STATES

Sets the number of excited state roots to find. For closed-shell reference,
defaults into EE_SINGLETS. For open-shell references, specifies all
low-lying states.

TYPE:

INTEGER/INTEGER ARRAY

DEFAULT:

0
Do not look for any excited states.

OPTIONS:

$[i,j,k\mathrm{\dots}]$
Find $i$ excited states in the first irrep, $j$ states
in the second irrep *etc.*

RECOMMENDATION:

None

EE_SINGLETS

Sets the number of singlet excited state roots to find. Valid only
for closed-shell references.

TYPE:

INTEGER/INTEGER ARRAY

DEFAULT:

0
Do not look for any excited states.

OPTIONS:

$[i,j,k\mathrm{\dots}]$
Find $i$ excited states in the first irrep, $j$ states
in the second irrep *etc.*

RECOMMENDATION:

None

EE_TRIPLETS

Sets the number of triplet excited state roots to find. Valid only
for closed-shell references.

TYPE:

INTEGER/INTEGER ARRAY

DEFAULT:

0
Do not look for any excited states.

OPTIONS:

$[i,j,k\mathrm{\dots}]$
Find $i$ excited states in the first irrep, $j$ states
in the second irrep *etc.*

RECOMMENDATION:

None

SF_STATES

Sets the number of spin-flip target states roots to find.

TYPE:

INTEGER/INTEGER ARRAY

DEFAULT:

0
Do not look for any excited states.

OPTIONS:

$[i,j,k\mathrm{\dots}]$
Find $i$ SF states in the first irrep, $j$ states
in the second irrep *etc.*

RECOMMENDATION:

None

DSF_STATES

Sets the number of doubly spin-flipped target states roots to find.

TYPE:

INTEGER/INTEGER ARRAY

DEFAULT:

0
Do not look for any DSF states.

OPTIONS:

$[i,j,k\mathrm{\dots}]$
Find $i$ doubly spin-flipped states in the first irrep, $j$ states
in the second irrep *etc.*

RECOMMENDATION:

None

IP_STATES

Sets the number of ionized target states roots to find. By default, $\beta $
electron will be removed (see EOM_IP_BETA).

TYPE:

INTEGER/INTEGER ARRAY

DEFAULT:

0
Do not look for any IP states.

OPTIONS:

$[i,j,k\mathrm{\dots}]$
Find $i$ ionized states in the first irrep, $j$ states
in the second irrep *etc.*

RECOMMENDATION:

None

EOM_IP_ALPHA

Sets the number of ionized target states derived by removing $\alpha $ electron
(M${}_{s}=-\frac{1}{2}$).

TYPE:

INTEGER/INTEGER ARRAY

DEFAULT:

0
Do not look for any IP/$\alpha $ states.

OPTIONS:

$[i,j,k\mathrm{\dots}]$
Find $i$ ionized states in the first irrep, $j$ states
in the second irrep *etc.*

RECOMMENDATION:

None

EOM_IP_BETA

Sets the number of ionized target states derived by removing $\beta $ electron
(M${}_{s}$=$\frac{1}{2}$, default for EOM-IP).

TYPE:

INTEGER/INTEGER ARRAY

DEFAULT:

0
Do not look for any IP/$\beta $ states.

OPTIONS:

$[i,j,k\mathrm{\dots}]$
Find $i$ ionized states in the first irrep, $j$ states
in the second irrep *etc.*

RECOMMENDATION:

None

EA_STATES

Sets the number of attached target states roots to find. By default, $\alpha $
electron will be attached (see EOM_EA_ALPHA).

TYPE:

INTEGER/INTEGER ARRAY

DEFAULT:

0
Do not look for any EA states.

OPTIONS:

$[i,j,k\mathrm{\dots}]$
Find $i$ EA states in the first irrep, $j$ states
in the second irrep *etc.*

RECOMMENDATION:

None

EOM_EA_ALPHA

Sets the number of attached target states derived by attaching $\alpha $
electron (M${}_{s}$=$\frac{1}{2}$, default in EOM-EA).

TYPE:

INTEGER/INTEGER ARRAY

DEFAULT:

0
Do not look for any EA states.

OPTIONS:

$[i,j,k\mathrm{\dots}]$
Find $i$ EA states in the first irrep, $j$ states
in the second irrep *etc.*

RECOMMENDATION:

None

EOM_EA_BETA

Sets the number of attached target states derived by attaching $\beta $
electron (M${}_{s}$=$-\frac{1}{2}$, EA-SF).

TYPE:

INTEGER/INTEGER ARRAY

DEFAULT:

0
Do not look for any EA states.

OPTIONS:

$[i,j,k\mathrm{\dots}]$
Find $i$ EA states in the first irrep, $j$ states
in the second irrep *etc.*

RECOMMENDATION:

None

DIP_STATES

Sets the number of DIP roots to find. For
closed-shell reference, defaults into DIP_SINGLETS. For open-shell references,
specifies all low-lying states.

TYPE:

INTEGER/INTEGER ARRAY

DEFAULT:

0
Do not look for any DIP states.

OPTIONS:

$[i,j,k\mathrm{\dots}]$
Find $i$ DIP states in the first irrep, $j$ states
in the second irrep *etc.*

RECOMMENDATION:

None

DIP_SINGLETS

Sets the number of singlet DIP roots to find. Valid only
for closed-shell references.

TYPE:

INTEGER/INTEGER ARRAY

DEFAULT:

0
Do not look for any singlet DIP states.

OPTIONS:

$[i,j,k\mathrm{\dots}]$
Find $i$ DIP singlet states in the first irrep, $j$ states
in the second irrep *etc.*

RECOMMENDATION:

None

DIP_TRIPLETS

Sets the number of triplet DIP roots to find. Valid only
for closed-shell references.

TYPE:

INTEGER/INTEGER ARRAY

DEFAULT:

0
Do not look for any DIP triplet states.

OPTIONS:

$[i,j,k\mathrm{\dots}]$
Find $i$ DIP triplet states in the first irrep, $j$ states
in the second irrep *etc.*

RECOMMENDATION:

None

DIP_AA_STATES

Sets the number of M${}_{s}$=-1 DIP roots (remove two $\alpha $ electrons) to find. Valid only
for closed-shell references.

TYPE:

INTEGER/INTEGER ARRAY

DEFAULT:

0
Do not look for any DIP M${}_{s}$=-1 states.

OPTIONS:

$[i,j,k\mathrm{\dots}]$
Find $i$ DIP states in the first irrep, $j$ states
in the second irrep *etc.*

RECOMMENDATION:

None

DIP_BB_STATES

Sets the number of M${}_{s}$=+1 DIP roots (remove two $\beta $ electrons) to find.

TYPE:

INTEGER/INTEGER ARRAY

DEFAULT:

0
Do not look for any DIP M${}_{s}$=+1 states.

OPTIONS:

$[i,j,k\mathrm{\dots}]$
Find $i$ DIP states in the first irrep, $j$ states
in the second irrep *etc.*

RECOMMENDATION:

None

DIP_AB_STATES

Sets the number of M${}_{s}$=0 DIP roots (remove one $\alpha $ and one $\beta $ electron) to find.

TYPE:

INTEGER/INTEGER ARRAY

DEFAULT:

0
Do not look for any DIP M${}_{s}$=0 states.

OPTIONS:

$[i,j,k\mathrm{\dots}]$
Find $i$ DIP states in the first irrep, $j$ states
in the second irrep *etc.*

RECOMMENDATION:

None

DEA_STATES

Sets the number of DEA roots to find. For
closed-shell reference, defaults into DEA_SINGLETS. For open-shell references,
specifies all low-lying states.

TYPE:

INTEGER/INTEGER ARRAY

DEFAULT:

0
Do not look for any DEA states.

OPTIONS:

$[i,j,k\mathrm{\dots}]$
Find $i$ DIP states in the first irrep, $j$ states
in the second irrep *etc.*

RECOMMENDATION:

None

DEA_SINGLETS

Sets the number of singlet DEA roots to find. Valid only
for closed-shell references.

TYPE:

INTEGER/INTEGER ARRAY

DEFAULT:

0
Do not look for any singlet DEA states.

OPTIONS:

$[i,j,k\mathrm{\dots}]$
Find $i$ DEA singlet states in the first irrep, $j$ states
in the second irrep *etc.*

RECOMMENDATION:

None

DEA_TRIPLETS

Sets the number of triplet DEA roots to find. Valid only
for closed-shell references.

TYPE:

INTEGER/INTEGER ARRAY

DEFAULT:

0
Do not look for any DEA triplet states.

OPTIONS:

$[i,j,k\mathrm{\dots}]$
Find $i$ DEA triplet states in the first irrep, $j$ states
in the second irrep *etc.*

RECOMMENDATION:

None

DEA_AA_STATES

Sets the number of M${}_{s}$=1 DEA roots (two $\alpha $ electrons) to find.

TYPE:

INTEGER/INTEGER ARRAY

DEFAULT:

0
Do not look for any DEA M${}_{s}$=1 transitions.

OPTIONS:

$[i,j,k\mathrm{\dots}]$
Find $i$ DEA $\alpha \alpha $ states in the first irrep, $j$ states
in the second irrep *etc.*

RECOMMENDATION:

None

DEA_BB_STATES

Sets the number of M${}_{s}$=-1 DEA roots (two $\beta $ electrons) to find.

TYPE:

INTEGER/INTEGER ARRAY

DEFAULT:

0
Do not look for any DEA M${}_{s}$=-1 transitions.

OPTIONS:

$[i,j,k\mathrm{\dots}]$
Find $i$ DEA $\beta \beta $ states in the first irrep, $j$ states
in the second irrep *etc.*

RECOMMENDATION:

None

DEA_AB_STATES

Sets the number of M${}_{s}$=0 DEA roots (one $\alpha $ and one $\beta $ electron) to find.

TYPE:

INTEGER/INTEGER ARRAY

DEFAULT:

0
Do not look for any DEA M${}_{s}$=0 transitions.

OPTIONS:

$[i,j,k\mathrm{\dots}]$
Find $i$ DEA $\alpha \beta $ states in the first irrep, $j$ states
in the second irrep *etc.*

RECOMMENDATION:

None

Note: It is a symmetry of a transition rather than that of a target state which is specified in excited state calculations. The symmetry of the target state is a product of the symmetry of the reference state and the transition. For closed-shell molecules, the former is fully symmetric and the symmetry of the target state is the same as that of transition, however, for open-shell references this is not so.

Note: For the XX_STATES options, Q-Chem will increase the number of roots if it suspects degeneracy, or change it to a smaller value, if it cannot generate enough guess vectors to start the calculations.

EOM_FAKE_IPEA

If TRUE, calculates fake EOM-IP or EOM-EA energies and properties
using the diffuse orbital trick. Default for EOM-EA and Dyson orbital
calculations in CCMAN.

TYPE:

LOGICAL

DEFAULT:

FALSE (use proper EOM-IP code)

OPTIONS:

FALSE, TRUE

RECOMMENDATION:

None. This feature only works for CCMAN.

Note:
When EOM_FAKE_IPEA is set to TRUE, it can change the
convergence of Hartree-Fock iterations compared to the same job without
EOM_FAKE_IPEA, because a very diffuse basis function is added to a
center of symmetry *before* the Hartree-Fock iterations start. For the
same reason, BASIS2 keyword is incompatible with
EOM_FAKE_IPEA. In order to read Hartree-Fock guess from a previous
job, you must specify EOM_FAKE_IPEA (even if you do not request for
any correlation or excited states) in that previous job. Currently, the second
moments of electron density and Mulliken charges and spin densities are
incorrect for the EOM-IP/EA-CCSD target states.

EOM_USER_GUESS

Specifies if user-defined guess will be used in EOM calculations.

TYPE:

LOGICAL

DEFAULT:

FALSE

OPTIONS:

TRUE
Solve for a state that has maximum overlap with a trans-n
specified in *$eom_user_guess*.

RECOMMENDATION:

The orbitals are ordered by energy, as printed in the beginning of the
CCMAN2 output. Not available in CCMAN.

EOM_SHIFT

Specifies energy shift in 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:

Not available in CCMAN.

EOM_NGUESS_DOUBLES

Specifies number of excited state guess vectors which are double excitations.

TYPE:

INTEGER

DEFAULT:

0

OPTIONS:

$n$
Include $n$ guess vectors that are double excitations

RECOMMENDATION:

This should be set to the expected number of doubly excited states,
otherwise they may not be found.

EOM_NGUESS_SINGLES

Specifies number of excited state guess vectors that are single excitations.

TYPE:

INTEGER

DEFAULT:

Equal to the number of excited states requested

OPTIONS:

$n$
Include $n$ guess vectors that are single excitations

RECOMMENDATION:

Should be greater or equal than the number of excited states requested, unless
.

EOM_PRECONV_SINGLES

When not zero, singly excited vectors are converged prior to a full excited
states calculation. Sets the maximum number of iterations for pre-converging
procedure.

TYPE:

INTEGER

DEFAULT:

0

OPTIONS:

0
do not pre-converge
1
pre-converge singles

RECOMMENDATION:

Sometimes helps with problematic convergence.

Note: In CCMAN, setting EOM_PRECONV_SINGLES = $N$ would result in $N$ Davidson iterations pre-converging singles.

EOM_PRECONV_DOUBLES

When not zero, doubly excited vectors are converged prior to a full excited
states calculation. Sets the maximum number of iterations for pre-converging procedure

TYPE:

INTEGER

DEFAULT:

0

OPTIONS:

0
Do not pre-converge
N
Perform N Davidson iterations pre-converging doubles.

RECOMMENDATION:

Occasionally necessary to ensure a doubly excited state is found. Also used in DSF, DIP, and DEA calculations
instead of EOM_PRECONV_SINGLES

Note: Not available for EOM-EE in CCMAN2.

EOM_PRECONV_SD

When not zero, EOM vectors are pre-converged prior to a full excited states
calculation. Sets the maximum number of iterations for pre-converging
procedure.

TYPE:

INTEGER

DEFAULT:

0

OPTIONS:

0
do not pre-converge
N
perform N Davidson iterations pre-converging singles and doubles.

RECOMMENDATION:

Occasionally necessary to ensure that all low-lying states are
found. Also, very useful in EOM(2,3) calculations.

None

Note: Not available in CCMAN2.

EOM_DAVIDSON_CONVERGENCE

Convergence criterion for the RMS residuals of excited state vectors.

TYPE:

INTEGER

DEFAULT:

5
Corresponding to ${10}^{-5}$

OPTIONS:

$n$
Corresponding to ${10}^{-n}$ convergence criterion

RECOMMENDATION:

Use the default. Normally this value be the same as EOM_DAVIDSON_THRESHOLD.

EOM_DAVIDSON_THRESHOLD

Specifies threshold for including a new expansion vector in the iterative
Davidson diagonalization. Their norm must be above this threshold.

TYPE:

INTEGER

DEFAULT:

00103
Corresponding to 0.00001

OPTIONS:

$abcde$
Integer code is mapped to $abc\times {10}^{-(de+2)}$, *i.e.*, 02505->2.5$\times {10}^{-6}$

RECOMMENDATION:

Use the default unless converge problems are encountered. Should normally be set
to the same values as EOM_DAVIDSON_CONVERGENCE, if convergence problems arise
try setting to a value slightly larger than EOM_DAVIDSON_CONVERGENCE.

EOM_DAVIDSON_MAXVECTORS

Specifies maximum number of vectors in the subspace for the Davidson
diagonalization.

TYPE:

INTEGER

DEFAULT:

60

OPTIONS:

$n$
Up to $n$ vectors per root before the subspace is reset

RECOMMENDATION:

Larger values increase disk storage but accelerate and stabilize convergence.

EOM_DAVIDSON_MAX_ITER

Maximum number of iteration allowed for Davidson diagonalization procedure.

TYPE:

INTEGER

DEFAULT:

30

OPTIONS:

$n$
User-defined number of iterations

RECOMMENDATION:

Default is usually sufficient

EOM_IPEA_FILTER

If TRUE, filters the EOM-IP/EA amplitudes obtained using the diffuse
orbital implementation (see EOM_FAKE_IPEA). Helps with convergence.

TYPE:

LOGICAL

DEFAULT:

FALSE (EOM-IP or EOM-EA amplitudes will not be filtered)

OPTIONS:

FALSE, TRUE

RECOMMENDATION:

None

Note: Not available in CCMAN2.

CC_FNO_THRESH

Initialize the FNO truncation and sets the threshold to be used for both cutoffs (OCCT and POVO).

TYPE:

INTEGER

DEFAULT:

None

OPTIONS:

range
0000-10000
$abcd$
Corresponding to $ab.cd$%

RECOMMENDATION:

None

CC_FNO_USEPOP

Selection of the truncation scheme.

TYPE:

INTEGER

DEFAULT:

1
OCCT

OPTIONS:

0
POVO

RECOMMENDATION:

None

SCALE_NUCLEAR_CHARGE

Scales charge of each nuclei by a certain value. The nuclear repulsion energy
is calculated for the unscaled nuclear charges.

TYPE:

INTEGER

DEFAULT:

0
No scaling.

OPTIONS:

$n$
A total positive charge of (1+$n/100$)e is added to the molecule.

RECOMMENDATION:

NONE

ADD_CHARGED_CAGE

Add a point charge cage of a given radius and total charge.

TYPE:

INTEGER

DEFAULT:

0
No cage.

OPTIONS:

0
No cage.
1
Dodecahedral cage.
2
Spherical cage.

RECOMMENDATION:

Spherical cage is expected to yield more accurate results, especially for small radii.

CAGE_RADIUS

Defines radius of the charged cage.

TYPE:

INTEGER

DEFAULT:

225

OPTIONS:

$n$
radius is $n/100$ Å.

RECOMMENDATION:

None

CAGE_POINTS

Defines number of point charges for the spherical cage.

TYPE:

INTEGER

DEFAULT:

100

OPTIONS:

$n$
Number of point charges to use.

RECOMMENDATION:

None

CAGE_CHARGE

Defines the total charge of the cage.

TYPE:

INTEGER

DEFAULT:

400
Add a cage charged +4e.

OPTIONS:

$n$
Total charge of the cage is $n/100$ a.u.

RECOMMENDATION:

None