Facing problem with Imaginary frequency

[Aarzoo]

Hi, I am calculating frequency for the first excited state using SF-DFT but getting one imaginary frequency. Can anyone suggest to me how to get rid of it?
Here is the input. Using an optimized z-matrix, I am calculating frequency.
$molecule
0 3
O
C 1 1.363660
O 2 1.226139 1 121.512800
C 1 1.429357 2 115.924844 3 -4.487271 0
H 4 1.095236 1 105.653988 2 -176.637884 0
H 4 1.099099 1 111.063620 2 -57.258099 0
H 4 1.099593 1 111.063063 2 64.231509 0
C 2 1.469618 1 110.966049 3 178.586023 0
C 8 1.414668 2 123.067752 1 -27.344672 0
H 9 1.092821 8 116.284110 2 159.255286 0
C 9 1.443284 8 127.673157 2 -27.171261 0
C 11 1.425518 9 119.695941 8 163.334612 0
H 12 1.091353 11 118.850438 9 -1.758165 0
C 12 1.380518 11 122.095547 9 178.857376 0
H 14 1.088193 12 118.816603 11 179.463465 0
C 11 1.430468 9 123.644720 8 -19.905651 0
H 16 1.089300 11 119.589180 9 -0.637843 0
C 16 1.381967 11 121.870983 9 -178.972216 0
H 18 1.088173 16 118.642616 11 -179.190198 0
C 18 1.425560 16 121.014571 11 0.554706 0
N 20 1.365227 18 121.468655 16 -178.360214 0
C 21 1.454657 20 120.389590 18 -1.028489 0
H 22 1.093208 21 109.124672 20 178.253925 0
H 22 1.101701 21 110.925630 20 58.659941 0
H 22 1.102033 21 111.111359 20 -61.860732 0
C 21 1.455874 20 120.751964 18 179.166180 0
H 26 1.093271 21 108.956413 20 -178.820115 0
H 26 1.101271 21 110.997900 20 -59.353094 0
H 26 1.101639 21 111.266573 20 61.600706 0
C 8 1.469511 2 120.598141 1 147.200712 0
O 30 1.230181 8 123.551766 2 178.459958 0
O 30 1.352630 8 114.686474 2 -4.191207 0
C 32 1.430640 30 115.881837 8 -178.409896 0
H 33 1.095081 32 105.510133 30 -179.090237 0
H 33 1.099390 32 111.150716 30 -59.841453 0
H 33 1.099438 32 111.010151 30 61.776384 0
$end

$rem
BASIS = cc-pVDZ
EXCHANGE = omegaB97XD
OMEGA = 104
JOB_TYPE = frequency
MAX_CIS_CYCLES = 700
SCF_CONVERGENCE = 8
MAX_SCF_CYCLES = 700
THRESH = 14
UNRESTRICTED = true
SPIN_FLIP = 1
CIS_N_ROOTS = 4
SYMMETRY_IGNORE = true
SYMMETRY = false
CIS_STATE_DERIVATIVE = 3
SOLVENT_METHOD = PCM
$end

$solvent
DIELECTRIC 32.613000
OPTICALDIELECTRIC 1.765709
$end

output:
STANDARD THERMODYNAMIC QUANTITIES AT 298.15 K AND 1.00 ATM

This Molecule has 1 Imaginary Frequencies
Zero point vibrational energy: 181.089 kcal/mol

Atom 1 Element O Has Mass 15.99491
Atom 2 Element C Has Mass 12.00000
Atom 3 Element O Has Mass 15.99491
Atom 4 Element C Has Mass 12.00000
Atom 5 Element H Has Mass 1.00783
Atom 6 Element H Has Mass 1.00783
Atom 7 Element H Has Mass 1.00783
Atom 8 Element C Has Mass 12.00000
Atom 9 Element C Has Mass 12.00000
Atom 10 Element H Has Mass 1.00783
Atom 11 Element C Has Mass 12.00000
Atom 12 Element C Has Mass 12.00000
Atom 13 Element H Has Mass 1.00783
Atom 14 Element C Has Mass 12.00000
Atom 15 Element H Has Mass 1.00783
Atom 16 Element C Has Mass 12.00000
Atom 17 Element H Has Mass 1.00783
Atom 18 Element C Has Mass 12.00000
Atom 19 Element H Has Mass 1.00783
Atom 20 Element C Has Mass 12.00000
Atom 21 Element N Has Mass 14.00307
Atom 22 Element C Has Mass 12.00000
Atom 23 Element H Has Mass 1.00783
Atom 24 Element H Has Mass 1.00783
Atom 25 Element H Has Mass 1.00783
Atom 26 Element C Has Mass 12.00000
Atom 27 Element H Has Mass 1.00783
Atom 28 Element H Has Mass 1.00783
Atom 29 Element H Has Mass 1.00783
Atom 30 Element C Has Mass 12.00000
Atom 31 Element O Has Mass 15.99491
Atom 32 Element O Has Mass 15.99491
Atom 33 Element C Has Mass 12.00000
Atom 34 Element H Has Mass 1.00783
Atom 35 Element H Has Mass 1.00783
Atom 36 Element H Has Mass 1.00783
Molecular Mass: 263.115820 amu
Principal axes and moments of inertia in amu*Bohr^2:
1 2 3
Eigenvalues – 2146.99441 10688.23005 12072.41997
X 0.19834 -0.95486 -0.22114
Y -0.67414 0.03088 -0.73796
Z 0.71148 0.29545 -0.63758
Rotational Symmetry Number is 1
The Molecule is an Asymmetric Top
Translational Enthalpy: 0.889 kcal/mol
Rotational Enthalpy: 0.889 kcal/mol
Vibrational Enthalpy: 191.665 kcal/mol
gas constant (RT): 0.592 kcal/mol
Translational Entropy: 42.602 cal/mol.K
Rotational Entropy: 33.982 cal/mol.K
Vibrational Entropy: 72.640 cal/mol.K

Total Enthalpy: 194.035 kcal/mol
Total Entropy: 149.223 cal/mol.K

Quasi-RRHO corrections using alpha = 4, and omega = 100 cm^-1
QRRHO-Vib. Enthalpy: 189.447 kcal/mol
QRRHO-Vib. Entropy: 64.321 cal/mol.K
QRRHO-Total Enthalpy: 191.817 kcal/mol
QRRHO-Total Entropy: 140.904 cal/mol.K

[jherbert]

Tighten up the geometry optimization thresholds, try to displace along the mode corresponding to the imaginary frequency. Unfortunately for a large molecule with SF-TDDFT. Alternatively (or in addition), compute the exact Hessian and read this in as a guess.

[jherbert]

Same recommendation (with some additional instructions) here:

How to troubleshoot artifical negative frequencies

I am doing rather routine calculation: TD-DFT, S1 state, optimization and frequencies. Everything converges smoothly, but then I end up with one imaginary frequency, which I am sure is an artefact. I tried several things at random with no effect. Question to the experts: What are standard steps of troubleshooting this sort of a problem? What are the first thing to tighten in terms of thresholds/convergence/grids? Here is my basic input, using defaults for everything: Blockquote $comment OPT&…

[Aarzoo]

Thanks @jherbert. I will try these recommendations.

[Aarzoo]

@jherbert I tried to tighten up the geometry optimization thresholds, and I got zero imaginary frequency. Thanks for your suggestion.