CDFT-CI Coupling convergence issue

Questions
#1

Hello, I am trying to perform a CDFT-CI calculation in Q-Chem 6.4 for an intramolecular donor–bridge-acceptor system that is an anion radical (overall charge = −1, multiplicity = 2). My goal is to model electron transfer between two fragments:

  • Donor region: atoms 1–21
  • Acceptor region: atoms 38–54
    I am having difficulty determining the correct CDFT-CI constraints. My current input file:
$molecule
-1 2
C         -1.05752       -1.39603        0.86890
C         -1.98340       -1.07935        1.87509
C         -3.34662       -1.24635        1.73247
C         -3.93541       -1.76228        0.52891
C         -2.97880       -2.08119       -0.49354
C         -1.62407       -1.89930       -0.31619
C         -5.34379       -1.95340        0.36716
C         -6.31069       -1.47018        1.31826
C         -7.66613       -1.66360        1.15836
C         -8.19340       -2.35200        0.05048
C         -7.27318       -2.83557       -0.89836
C         -5.91515       -2.64747       -0.75702
H         -1.58975       -0.70472        2.81804
H         -3.98378       -1.00946        2.57968
H         -3.32679       -2.46107       -1.44983
H         -0.94209       -2.15306       -1.12869
H         -5.97198       -0.90580        2.18253
H         -8.34434       -1.26407        1.91282
H         -9.26240       -2.50178       -0.06982
H         -7.63806       -3.37920       -1.77023
H         -5.25798       -3.06873       -1.51271
C          2.32798        0.56276        0.74525
C          3.14409       -0.64894        1.22539
C          2.49538       -1.29493        2.46371
C          0.95405       -1.19003        2.45442
C          0.44023       -1.18585        1.00607
C          0.89635        0.13018        0.31945
H          2.23755        1.24048        1.60426
H          4.17184       -0.34247        1.44840
H          3.21508       -1.37812        0.40891
H          2.79317       -2.34859        2.51161
H          2.88750       -0.82228        3.37161
H          0.64954       -0.26026        2.95605
H          0.52972       -2.01873        3.03046
H          0.95180       -2.01863        0.48928
H          0.84777        0.00310       -0.76789
H          0.20224        0.94114        0.56352
C          5.43790        5.25355       -1.94193
C          5.64548        4.63013       -3.19452
C          5.16854        3.36453       -3.42033
C          4.46393        2.66271       -2.40928
C          4.25120        3.28906       -1.15122
C          4.75746        4.59930       -0.94725
C          3.95839        1.35053       -2.60502
C          3.28103        0.70388       -1.60857
C          3.05532        1.31836       -0.34497
C          3.54208        2.58620       -0.14148
H          5.81806        6.25633       -1.77219
H          6.18181        5.15878       -3.97642
H          5.32100        2.88069       -4.38152
H          4.59600        5.07771        0.01512
H          4.11384        0.86582       -3.56523
H          2.90138       -0.29899       -1.78267
H          3.38478        3.07583        0.81721
$end

$rem
BASIS  = 6-31G**
METHOD  = wB97X-D
CDFTCI  = 1
CDFTCI_PRINT  = 2
GUI  = 2
JOB_TYPE  = SP
SCF_MAX_CYCLES  = 200
SYMMETRY  = FALSE
SYM_IGNORE = TRUE
UNRESTRICTED = TRUE
MEM_TOTAL = 248000
MEM_STATIC = 4000
$end

$cdft
  1.0
  1.0   1   21
  1.0
  1.0   1   21  s   
---
  1
  1.0   38  54
  1
  1.0   38  54  s  
$end

I am getting an “invalid charge and spin” error in the output file, so I am not sure whether my CDFT-CI constraints are defined correctly. Since this is a covalently linked donor–bridge–acceptor system, should I include the bridge atoms in the CDFT-CI regions, or is it sufficient to define only the donor (atoms 1–21) and acceptor (atoms 38–54) fragments?

Any guidance would be greatly appreciated.

#2

Should be sufficient to define either donor or acceptor, then the other is set to meet the total charge and multiplicity specified at the top of the $molecule section.

#3

I have tried several combinations—specifying both fragments, only the donor, and only the acceptor in the CDFT-CI block—but the job consistently crashes during the promolecule density construction for State 1 with the fatal error:

Invalid charge/spin combination in CDFT-CI block

Any insights would be greatly appreciated.`

$cdft
 1.0
 1.0   1  21
 1.0
 1.0   1  21 s
----------------
 0.0
 1.0   1  21
 0.0
 1.0   1  21 s
$end
#4

What about

$cdft
   1.0
   1.0   1   21
   0.0
   1.0   1   21 S
--------------
   0.0
   1.0   1   21
  -1.0
   1.0   1   21 S
$end

That seems to work for me, doesn’t generate that initial error and starts on the SCF. Here, I am copying from Example 5.5.28 in the manual,
https://manual.q-chem.com/latest/Ch5.S11.SS6.html

#5

The calculation ran successfully with the current setup. I have a question regarding the spin constraint on the first fragment in my CDFT-CI calculation. Since the system is an anion radical and the first fragment(donor) is assigned one excess electron, I expected this fragment to also carry one unit of excess spin. Could someone clarify how the spin constraint should be interpreted in this case?

For State 1, the Becke population analysis shows that the first fragment (donor) has approximately 0.69 excess electrons but only 0.21 excess spin. For State 2, the other fragment(acceptor) shows about 0.76 excess electrons and 1.08 excess spin. Why charge and spin value from becke population didn’t follow the constrained value?

#6

Looking at the original CDFT-CI paper,
https://doi.org/10.1063/1.3059784
I think that’s because the method uses what is essentially Hirshfeld-type fragment densities to define the constraints, which I don’t think is exactly the same as Becke partition so the Becke populations that are output may not strictly add to the desired constraints although they shouldn’t be wildly different. I believe we saw a similar effect in this paper,
https://pubs.acs.org/doi/10.1021/acs.jpca.0c11356