Hello, I am comparing S₁ geometry optimizations for a conjugated π-system (OPP) using:
-
TDDFT (CAM-B3LYP/6-31G(d,p), CPCM solvent)
-
ΔSCF with SGM (same functional, basis, and solvent)
Both calculations correspond to the lowest singlet excited state, which TDDFT indicates is predominantly a HOMO→LUMO π→π* excitation. Here is what I observe: -
The TDDFT S₁ optimization shows noticeable structural relaxation relative to S₀ (bond-length alternation changes and larger overall distortion).
-
The ΔSCF S₁ optimization results in much smaller geometry changes, even though:
- The occupation was enforced correctly (HOMO→LUMO),
- ⟨S²⟩ ≈ 0 (no spin contamination),
- SCF converged cleanly using SGM.
The vertical excitation energies from TDDFT and ΔSCF are reasonably close (3.4 eV and 3.3 eV), but the optimized geometries differ significantly in terms of relaxation magnitude. My questions: Is it expected that ΔSCF may underestimate structural relaxation compared to TDDFT for local π→π* excitations? And what would be the correct approach to handle this situation?
I am sharing both the input file:
TDDFT:
$molecule
0 1
H 1.3292676 -1.6836893 -3.0809952
C 0.7356205 -0.9500791 -3.6169987
C 0.0000000 0.0000000 -2.8999730
C -0.7356205 0.9500791 -3.6169987
H -1.3292676 1.6836893 -3.0809952
C -0.7357354 0.9503581 -5.0068087
H -1.3188870 1.6920649 -5.5435948
C 0.0000000 0.0000000 -5.7078619
H 0.0000000 0.0000000 -6.7931470
C 0.7357354 -0.9503581 -5.0068087
H 1.3188870 -1.6920649 -5.5435948
C 0.0000000 0.0000000 -1.4156563
C -0.0005202 1.1978303 -0.6937517
C 0.0005202 1.1978303 0.6937517
C 0.0000000 0.0000000 1.4156563
C -0.0005202 -1.1978303 0.6937517
C 0.0005202 -1.1978303 -0.6937517
C 0.0000000 0.0000000 2.8999730
C 0.7356205 0.9500791 3.6169987
C 0.7357354 0.9503581 5.0068087
C 0.0000000 0.0000000 5.7078619
C -0.7357354 -0.9503581 5.0068087
C -0.7356205 -0.9500791 3.6169987
H 0.0109345 2.1444854 -1.2243742
H -0.0109345 2.1444854 1.2243742
H 0.0109345 -2.1444854 1.2243742
H -0.0109345 -2.1444854 -1.2243742
H 1.3292676 1.6836893 3.0809952
H 1.3188870 1.6920649 5.5435948
H 0.0000000 0.0000000 6.7931470
H -1.3188870 -1.6920649 5.5435948
H -1.3292676 -1.6836893 3.0809952
$end
$rem
BASIS = 6-31G(d,p)
GUI = 2
JOB_TYPE = Optimization
GEOM_OPT_MAX_CYCLES = 200
METHOD = CAMB3LYP
SCF_CONVERGENCE = 8
SCF_MAX_CYCLES = 100
CIS_N_ROOTS = 3
CIS_SINGLETS = TRUE
CIS_TRIPLETS = FALSE
RPA = TRUE
CIS_STATE_DERIV = 1
SOLVENT_METHOD = PCM
SYMMETRY = FALSE
SYM_IGNORE = TRUE
MEM_TOTAL = 248000
MEM_STATIC = 4000
$end
$pcm
THEORY CPCM
heavypoints 590
method swig
radii bondi
solver inversion
$end
$solvent
DIELECTRIC 37.5
OPTICALDIELECTRIC 1.8068
$end
delSCF:
$molecule
0 1
H 1.3292676 -1.6836893 -3.0809952
C 0.7356205 -0.9500791 -3.6169987
C 0.0000000 0.0000000 -2.8999730
C -0.7356205 0.9500791 -3.6169987
H -1.3292676 1.6836893 -3.0809952
C -0.7357354 0.9503581 -5.0068087
H -1.3188870 1.6920649 -5.5435948
C 0.0000000 0.0000000 -5.7078619
H 0.0000000 0.0000000 -6.7931470
C 0.7357354 -0.9503581 -5.0068087
H 1.3188870 -1.6920649 -5.5435948
C 0.0000000 0.0000000 -1.4156563
C -0.0005202 1.1978303 -0.6937517
C 0.0005202 1.1978303 0.6937517
C 0.0000000 0.0000000 1.4156563
C -0.0005202 -1.1978303 0.6937517
C 0.0005202 -1.1978303 -0.6937517
C 0.0000000 0.0000000 2.8999730
C 0.7356205 0.9500791 3.6169987
C 0.7357354 0.9503581 5.0068087
C 0.0000000 0.0000000 5.7078619
C -0.7357354 -0.9503581 5.0068087
C -0.7356205 -0.9500791 3.6169987
H 0.0109345 2.1444854 -1.2243742
H -0.0109345 2.1444854 1.2243742
H 0.0109345 -2.1444854 1.2243742
H -0.0109345 -2.1444854 -1.2243742
H 1.3292676 1.6836893 3.0809952
H 1.3188870 1.6920649 5.5435948
H 0.0000000 0.0000000 6.7931470
H -1.3188870 -1.6920649 5.5435948
H -1.3292676 -1.6836893 3.0809952
$end
$rem
JOB_TYPE SP
METHOD CAMB3LYP
BASIS 6-31G(d,p)
SCF_ALGORITHM DIIS
SCF_CONVERGENCE 8
MAX_SCF_CYCLES 200
SYM_IGNORE TRUE
SOLVENT_METHOD PCM
MEM_TOTAL 248000
MEM_STATIC 4000
$end
$pcm
THEORY CPCM
heavypoints 590
method swig
radii bondi
solver inversion
$end
$solvent
DIELECTRIC 37.5
OPTICALDIELECTRIC 1.8068
$end
@@@
$molecule
read
$end
$rem
JOB_TYPE OPTIMIZATION
METHOD CAMB3LYP
BASIS 6-31G(d,p)
UNRESTRICTED TRUE
SCF_ALGORITHM SGM_LS
SCF_CONVERGENCE 6
SCF_GUESS READ
MAX_SCF_CYCLES 200
SYM_IGNORE TRUE
SOLVENT_METHOD PCM
MEM_TOTAL 248000
MEM_STATIC 4000
$end
$pcm
THEORY CPCM
heavypoints 590
method swig
radii bondi
solver inversion
$end
$solvent
DIELECTRIC 37.5
OPTICALDIELECTRIC 1.8068
$end
$occupied
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 62
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61
$end