Hi everyone,
I have a conceptual question regarding the applicability of SF-TDDFT method.
I am studying a benzene + O(¹D) system, where benzene is in its closed-shell singlet ground state, and the oxygen atom is in its excited singlet ¹D state. At large separation, the overall system is therefore singlet–singlet.
My question is:
Can this system be meaningfully treated using the SF-TDDFT framework?
As I understand it, SF-TDDFT requires a high-spin reference state (e.g., a triplet), from which lower-spin states are accessed by spin-flip excitations. However, in the benzene + O(¹D) case, both fragments are singlets.
Is it inappropriate to use SF-TDDFT for this system, at least in the non-interacting or weakly interacting regime? If so, would conventional TDDFT or a multireference approach be more appropriate for describing excited-state dynamics?
I would appreciate any clarification or guidance on how one should properly set up excited-state calculations for this type of system in Q-Chem.
Thank you very much.
I tried to put a job as well:
input:
$molecule
0 3
H 1.954110 1.501681 -0.000017
C 1.099407 0.844699 -0.000021
C 1.281178 -0.530526 -0.000028
H 2.277161 -0.943033 -0.000032
C 0.181436 -1.373577 -0.000037
H 0.322351 -2.442191 -0.000046
C -1.099388 -0.844754 -0.000030
H -1.954087 -1.501741 -0.000034
C -1.281160 0.530470 -0.000016
H -2.277141 0.942980 -0.000009
C -0.181409 1.373524 -0.000016
H -0.322324 2.442138 -0.000008
O 0.000022 -0.000010 -3.000025
$end
$rem
BASIS = 6-31G*
JOB_TYPE = sp
METHOD = BHHLYP
SYMMETRY_IGNORE = TRUE
MAX_CIS_CYCLES = 500
MAX_SCF_CYCLES = 500
THRESH = 14
SPIN_FLIP = true
CIS_N_ROOTS = 6
CIS_STATE_DERIVATIVE = 1
sts_mom = true
UNRESTRICTED = true
SCF_CONVERGENCE = 8
$end
and output is:
SF-DFT Excitation Energies
(The first “excited” state might be the ground state)
Excited state 1: excitation energy (eV) = 1.6066
Total energy for state 1: -307.08509720 au
<S**2> : 2.0054
D( 16) → S( 1) amplitude = 0.6923
D( 17) → S( 2) amplitude = -0.6924
Excited state 2: excitation energy (eV) = 2.6689
Total energy for state 2: -307.04605844 au
<S**2> : 0.0190
D( 16) → S( 1) amplitude = -0.2698
D( 16) → S( 2) amplitude = 0.6312
D( 17) → S( 1) amplitude = -0.6326
D( 17) → S( 2) amplitude = -0.2700
Excited state 3: excitation energy (eV) = 2.6689
Total energy for state 3: -307.04605836 au
<S**2> : 0.0190
D( 16) → S( 1) amplitude = -0.6320
D( 16) → S( 2) amplitude = -0.2697
D( 17) → S( 1) amplitude = 0.2701
D( 17) → S( 2) amplitude = -0.6317
Excited state 4: excitation energy (eV) = 3.7159
Total energy for state 4: -307.00758397 au
<S**2> : 0.2174
D( 16) → S( 2) amplitude = -0.5899
D( 17) → S( 1) amplitude = -0.5868
S( 1) → S( 2) amplitude = 0.3663 alpha
S( 2) → S( 1) amplitude = 0.3726 alpha
Excited state 5: excitation energy (eV) = 4.1273
Total energy for state 5: -306.99246574 au
<S**2> : 1.0257
S( 1) → S( 1) amplitude = -0.6000 alpha
S( 1) → S( 2) amplitude = -0.1699 alpha
S( 2) → S( 1) amplitude = -0.2185 alpha
S( 2) → S( 2) amplitude = 0.7367 alpha
Excited state 6: excitation energy (eV) = 4.1631
Total energy for state 6: -306.99114905 au
<S**2> : 1.0124
S( 1) → S( 1) amplitude = 0.2073 alpha
S( 1) → S( 2) amplitude = -0.6416 alpha
S( 2) → S( 1) amplitude = 0.6714 alpha
S( 2) → S( 2) amplitude = 0.2201 alpha