7.8 Coupled-Cluster Excited-State and Open-Shell Methods

7.8.13 Equation-of-Motion Coupled-Cluster Job Control

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 6.14).

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 C1 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.

7.8.13.1 Alternative way to set up EOM calculations

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 7.1: Default and allowed CORRELATION and EOM_CORR combinations as well as valid target state types. The last column shows if a method is available in CCMAN or CCMAN2.

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_SNGLETS, EE_TRIPLETS
SF-CIS CI CIS SF_STATES
CIS(D) CI CIS(D) EE_STATES
EE_SNGLETS, EE_TRIPLETS
SF-CIS(D) CI CIS(D) SF_STATES
CISD CI CISD EE_STATES
EE_SNGLETS, EE_TRIPLETS
SF-CISD CI CISD SF_STATES
IP-CISD CI CISD IP_STATES
CISDT CI SDT EE_STATES
EE_SNGLETS, EE_TRIPLETS
SF-CISDT CI SDT or DT SF_STATES
EOM-EE-CCSD CCSD EE_STATES
EE_SNGLETS, EE_TRIPLETS
EOM-SF-CCSD CCSD SF_STATES
EOM-IP-CCSD CCSD IP_STATES
EOM-EA-CCSD CCSD EA_STATES
EOM-DIP-CCSD CCSD DIP_STATES
DIP_SNGLETS, DIP_TRIPLETS
EOM-2SF-CCSD CCSD SDT or DT DSF_STATES
EOM-EE-(2,3) CCSD SDT EE_STATES
EE_SNGLETS, 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
Table 7.2: Commonly used EOM and CI models. ’SINGLETS’ and ’TRIPLETS’ are only available for closed-shell references.

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] 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] 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] 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] 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] 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, β 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] 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 α electron (M=s-12).
TYPE:
       INTEGER/INTEGER ARRAY
DEFAULT:
       0 Do not look for any IP/α states.
OPTIONS:
       [i,j,k] 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 β electron (Ms=12, default for EOM-IP).
TYPE:
       INTEGER/INTEGER ARRAY
DEFAULT:
       0 Do not look for any IP/β states.
OPTIONS:
       [i,j,k] 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, α 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] 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 α electron (Ms=12, default in EOM-EA).
TYPE:
       INTEGER/INTEGER ARRAY
DEFAULT:
       0 Do not look for any EA states.
OPTIONS:
       [i,j,k] 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 β electron (Ms=-12, EA-SF).
TYPE:
       INTEGER/INTEGER ARRAY
DEFAULT:
       0 Do not look for any EA states.
OPTIONS:
       [i,j,k] 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] 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] 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] Find i DIP triplet 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 n10-3 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 calculations instead of EOM_PRECONV_SINGLES

Note:  Not available 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×10-(de+2), i.e., 02505->2.5×10-6
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