5.13 Methods Based on “Constrained” DFT

5.13.4 CDFT-CI Job Control and Examples

CDFTCI
       Initiates a constrained DFT-configuration interaction calculation
TYPE:
       LOGICAL
DEFAULT:
       FALSE
OPTIONS:
       TRUE Perform a CDFT-CI Calculation FALSE No CDFT-CI
RECOMMENDATION:
       Set to TRUE if a CDFT-CI calculation is desired.

CDFTCI_PRINT
       Controls level of output from CDFT-CI procedure to Q-Chem output file.
TYPE:
       INTEGER
DEFAULT:
       0
OPTIONS:
       0 Only print energies and coefficients of CDFT-CI final states 1 Level 0 plus CDFT-CI overlap, Hamiltonian, and population matrices 2 Level 1 plus eigenvectors and eigenvalues of the CDFT-CI population matrix 3 Level 2 plus promolecule orbital coefficients and energies
RECOMMENDATION:
       Level 3 is primarily for program debugging; levels 1 and 2 may be useful for analyzing the coupling elements

CDFT_LAMBDA_MODE
       Allows CDFT potentials to be specified directly, instead of being determined as Lagrange multipliers.
TYPE:
       BOOLEAN
DEFAULT:
       FALSE
OPTIONS:
       FALSE Standard CDFT calculations are used. TRUE Instead of specifying target charge and spin constraints, use the values from the input deck as the value of the Becke weight potential
RECOMMENDATION:
       Should usually be set to FALSE. Setting to TRUE can be useful to scan over different strengths of charge or spin localization, as convergence properties are improved compared to regular CDFT(-CI) calculations.

CDFTCI_SKIP_PROMOLECULES
       Skips promolecule calculations and allows fractional charge and spin constraints to be specified directly.
TYPE:
       BOOLEAN
DEFAULT:
       FALSE
OPTIONS:
       FALSE Standard CDFT-CI calculation is performed. TRUE Use the given charge/spin constraints directly, with no promolecule calculations.
RECOMMENDATION:
       Setting to TRUE can be useful for scanning over constraint values.

Note:  CDFT_LAMBDA_MODE and CDFTCI_SKIP_PROMOLECULES are mutually incompatible.

CDFTCI_SVD_THRESH
       By default, a symmetric orthogonalization is performed on the CDFT-CI matrix before diagonalization. If the CDFT-CI overlap matrix is nearly singular (i.e., some of the diabatic states are nearly degenerate), then this orthogonalization can lead to numerical instability. When computing 𝐒-1/2, eigenvalues smaller than 10-CDFTCI_SVD_THRESH are discarded.
TYPE:
       INTEGER
DEFAULT:
       4
OPTIONS:
       n for a threshold of 10-n.
RECOMMENDATION:
       Can be decreased if numerical instabilities are encountered in the final diagonalization.

CDFTCI_STOP
       The CDFT-CI procedure involves performing independent SCF calculations on distinct constrained states. It sometimes occurs that the same convergence parameters are not successful for all of the states of interest, so that a CDFT-CI calculation might converge one of these diabatic states but not the next. This variable allows a user to stop a CDFT-CI calculation after a certain number of states have been converged, with the ability to restart later on the next state, with different convergence options.
TYPE:
       INTEGER
DEFAULT:
       0
OPTIONS:
       n Stop after converging state n (the first state is state 1) 0 Do not stop early
RECOMMENDATION:
       Use this setting if some diabatic states converge but others do not.

CDFTCI_RESTART
       To be used in conjunction with CDFTCI_STOP, this variable causes CDFT-CI to read already-converged states from disk and begin SCF convergence on later states. Note that the same $cdft section must be used for the stopped calculation and the restarted calculation.
TYPE:
       INTEGER
DEFAULT:
       0
OPTIONS:
       n Start calculations on state n+1
RECOMMENDATION:
       Use this setting in conjunction with CDFTCI_STOP.

Example 5.25  CDFT-CI calculation of couplings between the anionic GFP chromophore (CHR:1-27) and a tyrosine (TYR:28-43) residue. The two diabatic states are Chro-(MS=0)Tyr(MS=0) and Chro(MS=1/2)Tyr-(MS=1/2).

$molecule
   -1 1
   C        -1.453000       -1.953000       -0.264000
   N        -0.278000       -1.402000       -0.440000
   N        -1.804000       -2.052000        1.091000
   C        -0.687000       -1.548000        1.806000
   O        -0.688000       -1.514000        3.031000
   C         0.291000       -1.140000        0.799000
   C         1.500000       -0.563000        1.254000
   H         1.585000       -0.660000        2.346000
   C         2.608000        0.030000        0.605000
   C         2.763000        0.182000       -0.865000
   H         1.926000       -0.073000       -1.543000
   C         3.733000        0.548000        1.313000
   H         3.682000        0.571000        2.326000
   C         3.821000        0.875000       -1.473000
   H         3.844000        1.102000       -2.575000
   C         4.938000        1.111000        0.700000
   H         5.734000        1.441000        1.308000
   C         5.037000        1.228000       -0.739000
   O         6.011000        1.818000       -1.261000
   C        -3.000000       -2.533000        1.832000
   H        -2.859000       -2.250000        2.892000
   H        -3.829000       -2.121000        1.354000
   C        -2.373000       -2.282000       -1.448000
   H        -1.790000       -3.026000       -2.045000
   H        -2.626000       -1.300000       -1.865000
   H        -3.054000       -3.631000        1.855000
   H        -3.308000       -2.854000       -1.357000
   C         7.648000       -5.429000        0.303000
   H         8.028000       -4.514000        0.845000
   H         7.274000       -5.098000       -0.671000
   C         6.499001       -5.986000        1.016000
   C         6.462999       -6.032001        2.390000
   H         7.284000       -5.579000        2.957000
   C         5.243000       -6.435000        3.018000
   H         5.190001       -6.315001        4.035000
   C         4.242001       -7.048000        2.189000
   O         3.095000       -7.615000        2.715000
   H         2.500999       -7.869000        1.979000
   C         5.454000       -6.469000        0.200000
   H         5.565001       -6.363000       -0.835000
   C         4.294001       -7.003000        0.803000
   H         3.469000       -7.324000        0.139000
   H         8.511000       -6.108000        0.245000
$end

$rem
   SYMMETRY         = off
   SYM_IGNORE       = true
   METHOD           = b3lyp
   BASIS            = cc-pvdz
   UNRESTRICTED     = true
   SCF_CONVERGENCE  =  8
   MAX_SCF_CYCLES   = 200
   CDFTCI           = true
   CDFTCI_PRINT     = 2
   CDFT_THRESH      = 7
$end

$cdft
   1.0
   1.0   1   27
   0.0
   1.0   1   27 S
--------------
   0.0
   1.0   1   27
  -1.0
   1.0   1   27 S
$end