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& Frequency calculation for

S1 structure optimized with

method = wB97X-D

basis = 6-311+G**

Also, use old optimzier

Start with slightly distorted structure to see if there is lower-energy

minimum compared to D2 struture

$end

$molecule

0 1

C 1.4029162007 1.3106017388 0.0000590696

C 0.7000528271 1.4568212458 1.2118385596

H 1.2444817885 1.4462938585 2.1496228374

C -0.7002534075 1.4565647257 1.2117792977

H -1.2447241991 1.4459115552 2.1495365571

C -1.4029162007 1.3106017388 -0.0000590696

C -0.7000528271 1.4568212458 -1.2118385596

H -1.2444817885 1.4462938585 -2.1496228374

C 0.7002534075 1.4565647257 -1.2117792977

H 1.2447241991 1.4459115552 -2.1495365571

C 2.8207494939 0.7871832610 0.0003491932

H 3.3544685496 1.1572220091 0.8803024505

H 3.3551773248 1.1579436494 -0.8789103039

C -2.8207494939 0.7871832610 -0.0003491932

H -3.35 1.1572220091 -0.8803024505

H -3.36 1.1579436494 0.8789103039

C 2.8207494939 -0.7871832610 -0.0003491932

H 3.3544685496 -1.1572220091 -0.8803024505

H 3.3551773248 -1.1579436494 0.8789103039

C 1.4029162007 -1.3106017388 -0.0000590696

C 0.7000528271 -1.4568212458 -1.2118385596

H 1.2444817885 -1.4462938585 -2.1496228374

C -0.7002534075 -1.4565647257 -1.2117792977

H -1.2447241991 -1.4459115552 -2.1495365571

C -1.4029162007 -1.3106017388 0.0000590696

C -0.7000528271 -1.4568212458 1.2118385596

H -1.2444817885 -1.4462938585 2.1496228374

C 0.7002534075 -1.4565647257 1.2117792977

H 1.2447241991 -1.4459115552 2.1495365571

C -2.8207494939 -0.7871832610 0.0003491932

H -3.3544685496 -1.1572220091 0.8803024505

H -3.3551773248 -1.1579436494 -0.8789103039

$end

$rem

jobtype = opt

geom_opt_driver optimize !use old algorithm

method = wB97X-D

basis = 6-311+G**

cis_n_roots = 1

cis_state_deriv = 1

cis_triplets=false

cis_singlets=true

mem_total = 10000

!XC_GRID = 3 !use SG3 grid.

$end

@@@

$molecule

read

$end

$rem

jobtype = freq

method = wB97X-D

basis = 6-311+G**

!XC_GRID = 3 !use SG3 grid

scf_guess = read

cis_n_roots = 1

cis_state_deriv = 1

cis_triplets=false

cis_singlets=true

mem_total = 10000

$end