Hello,

I’ve been trying to optimize the S1 structure of [2,2]-paracyclophane using TD-DFT. For some unknown reason I keep getting a a single negative frequency in the normal mode analysis (indicating a saddle point). Iv’e tried everything, starting from ignoring symmetry in calculations, distorting the molecule along this negative mode, and reloading the hessian from the previous calculation for further iterations, but nothing seems to work. I also tried to change the XC grid.

It is worth saying that when using a simple 6-31G(d) basis, the molecule optimizes into a local minimum, resulting with all positive frequencies. The problem begins, when I use a the 6-31+G(d) basis.

Here is my input file:

$molecule

0 1

C -1.40512 -1.30536 0.14389

C -0.82457 -1.45948 -1.14380

H -1.46326 -1.47782 -2.02917

C 0.57679 -1.43544 -1.27970

H 1.02762 -1.40777 -2.27299

C 1.40514 -1.30533 -0.14389

C 0.82460 -1.45947 1.14380

H 1.46329 -1.47779 2.02917

C -0.57676 -1.43546 1.27970

H -1.02759 -1.40780 2.27299

C -2.82381 -0.78677 0.26176

H -3.16214 -0.87195 1.37988

H -3.55773 -1.44342 -0.38083

C 2.82383 -0.78671 -0.26176

H 3.16217 -0.87187 -1.37988

H 3.55775 -1.44335 0.38083

C 2.82381 0.78677 0.26176

H 3.16214 0.87195 1.37987

H 3.55772 1.44342 -0.38083

C 1.40512 1.30536 0.14389

C 0.82457 1.45948 -1.14380

H 1.46326 1.47782 -2.02917

C -0.57679 1.43544 -1.27970

H -1.02762 1.40777 -2.27299

C -1.40514 1.30533 -0.14389

C -0.82460 1.45947 1.14380

H -1.46329 1.47779 2.02917

C 0.57676 1.43545 1.27970

H 1.02759 1.40780 2.27299

C -2.82383 0.78671 -0.26176

H -3.16217 0.87187 -1.37988

H -3.55775 1.44335 0.38083

$end

$rem

JOBTYPE opt

EXCHANGE CAMB3LYP

BASIS 6-31+G(d)

DFT_D D4

CIS_STATE_DERIV 1

CIS_N_ROOTS 5

CIS_SINGLETS true

CIS_TRIPLETS false

RPA false

SYMMETRY false

SYM_IGNORE true

GEOM_OPT_TOL_DISPLACEMENT 10

GEOM_OPT_TOL_ENERGY 10

GEOM_OPT_TOL_GRADIENT 10

THRESH 14

GEOM_OPT_MAX_CYCLES 100

XC_GRID = 000075000302

mem_total 12000

$end

@@@

$molecule

read

$end

$rem

JOBTYPE freq

EXCHANGE CAMB3LYP

BASIS 6-31+G(d)

DFT_D D4

CIS_STATE_DERIV 1

CIS_N_ROOTS 2

CIS_SINGLETS true

CIS_TRIPLETS false

RPA false

mem_total 12000

SYM_IGNORE true

SYMMETRY false

XC_GRID = 000075000302

$end

@@@

$molecule

read

$end

$rem

JOBTYPE opt

GEOM_OPT_HESSIAN read

EXCHANGE CAMB3LYP

BASIS 6-31+G(d)

DFT_D D4

CIS_STATE_DERIV 1

CIS_N_ROOTS 5

CIS_SINGLETS true

CIS_TRIPLETS false

RPA false

SYMMETRY false

SYM_IGNORE true

GEOM_OPT_TOL_DISPLACEMENT 10

GEOM_OPT_TOL_ENERGY 10

GEOM_OPT_TOL_GRADIENT 10

THRESH 14

GEOM_OPT_MAX_CYCLES 100

XC_GRID = 000075000302

mem_total 12000

$end

@@@

$molecule

read

$end

$rem

JOBTYPE freq

EXCHANGE CAMB3LYP

BASIS 6-31+G(d)

DFT_D D4

CIS_STATE_DERIV 1

CIS_N_ROOTS 2

CIS_SINGLETS true

CIS_TRIPLETS false

RPA false

mem_total 12000

SYM_IGNORE true

SYMMETRY false

XC_GRID = 000075000302

$end

Thanks!

Omer.