Relax PES scan in Q chem

Questions
#1

I am running relax PES scan of passing proton through the center of Benzene from 2.2 Angstrom to -0.8 Angstrom. I have Dummy atom placed at a -0.8 Angstrom along the Z axis. I am receiving the following error. Can any give me an idea how to solve this. The input and error is given below
$molecule
1 1
C 1.22127000 0.70510000 0.00000000
C 1.22127000 -0.70510000 0.00000000
C 0.00000000 1.41021000 0.00000000
C -1.22127000 -0.70510000 -0.00000000
C -0.00000000 -1.41021000 -0.00000000
C -1.22127000 0.70510000 -0.00000000
H 2.15657900 1.24509729 0.00000000
H 2.15657900 -1.24509729 0.00000000
H 0.00000000 2.49021000 0.00000000
H -2.15657900 -1.24509729 -0.00000000
H -0.00000000 -2.49021000 -0.00000000
H -2.15657900 1.24509729 -0.00000000
H 0 0 2.2
X 0 0 -0.8
$end

$FIXED
14 0 0 -0.8
$ENDFXIED

$rem
BASIS = def2-TZVP
JOBTYPE = PES_SCAN
MAX_SCF_CYCLES = 100
METHOD = B3LYP
SCF_ALGORITHM = GDM
SCF_CONVERGENCE = 5
THRESH = 14
$end

$scan
stre 13 14 -0.8 2.2 0.1
$end
Error: ERROR Distance Constraint must be positive

Q-Chem fatal error occurred in module 0, line 9:

OPTIMIZE fatal error

#2

I think it’s telling you that the “stre” type is a bond length, not an absolute coordinate, so it cannot be negative.

#3

So what could be the possible solution?

#4

What Prof. Herbert is suggesting is “-0.8” in your $scan cannot be a negative number, because a bond length cannot be negative.

#5

Examples here:
https://manual.q-chem.com/6.2/Ch9.S6.SS1.html

#6

Thanks for your reply. I do know how to do PES_SCAN. But since my proton has to pass through the center of the benzene ring from +Z direction towards - Z direction, for that I have to define Dummy atom which makes it complicated. I am constantly getting error that Dummy atom has to be part of the constraint which it is already. If you know how to deal with that, please let me know. Thanks
$molecule
1 1
C 1.22127000 0.70510000 0.00000000
C 1.22127000 -0.70510000 0.00000000
C 0.00000000 1.41021000 0.00000000
C -1.22127000 -0.70510000 -0.00000000
C -0.00000000 -1.41021000 -0.00000000
C -1.22127000 0.70510000 -0.00000000
H 2.15657900 1.24509729 0.00000000
H 2.15657900 -1.24509729 0.00000000
H 0.00000000 2.49021000 0.00000000
H -2.15657900 -1.24509729 -0.00000000
H -0.00000000 -2.49021000 -0.00000000
H -2.15657900 1.24509729 -0.00000000
H 0 0 2.2
$end

$dummy
14 2 3 13
$enddummy

$rem
BASIS = def2-TZVP
JOBTYPE = PES_SCAN
MAX_SCF_CYCLES = 100
METHOD = B3LYP
SCF_ALGORITHM = GDM
$end
$scan
stre 13 14 0.0 3.0 0.1

#7

If you’re trying to put a proton atop benzene, why not just do it on one side and then flip everything by symmetry?

#8

By this do you mean that I should remove the dummy atom and scan it just from one side? If yes then How I will let the proton go through the center of the benzene ring?

#9

It has C6v symmetry, it’s the same on both sides. More to the point, you can probably do this even without constraints, due to the symmetry. GEOM_OPT_DRIVER = OPTIMIZE is better at preserving the initial point-group symmetry.