Overtones and Combination tones

Questions
#1

I would like to get the overtones and the combination tones, but I can’t get them by anharmonic calculation. if I use Gaussian, I think I can calculate the overtones and the combination tones by anharmonic calculation, but in Q-Chem, how should I calculate them?
Any advice you can give us would be greatly appreciated.

The following is the actual input file used in the calculation.

$molecule
0 1
C -4.3555400 0.6876800 0.0030300
C -3.4738600 -0.5508400 -0.0085800
H -4.1530300 1.3221800 -0.8657400
H -4.1873200 1.2799600 0.9081500
H -5.4116000 0.4014400 -0.0239900
C -1.9916500 -0.1828600 0.0125000
H -3.6972700 -1.1432100 -0.9035200
H -3.7164100 -1.1730100 0.8608500
C -1.1101600 -1.4215700 0.0168400
H -1.7692300 0.4199200 0.9006700
H -1.7479400 0.4290500 -0.8639100
H -1.3002400 -2.0346600 0.9036400
H -0.0538300 -1.1350800 0.0222500
H -1.2912400 -2.0356600 -0.8710600
$end

$rem
METHOD b3lyp
BASIS 6-31G(d)

UNRESTRICTED 1
SCF_ALGORITHM = diis_gdm
MAX_DIIS_CYCLES = 1
SCF_GUESS = gwh
SCF_GUESS_MIX = 2
MAX_SCF_CYCLES 300

JOBTYPE opt
GEOM_OPT_DMAX 50
GEOM_OPT_MAX_CYCLES 1000
GEOM_OPT_CHARAC TRUE

SYMMETRY false
SYM_IGNORE true
integral_symmetry false
point_group_symmetry False

$end

@@@

$molecule
read
$end

$rem
METHOD b3lyp
BASIS 6-31g(d)

UNRESTRICTED 1
SCF_ALGORITHM = diis_gdm
MAX_DIIS_CYCLES = 1
SCF_GUESS = read
SCF_GUESS_MIX = 2
MAX_SCF_CYCLES 300

JOBTYPE freq
ANHAR TRUE
MODE_COUPLING 3

SYMMETRY false
SYM_IGNORE true
integral_symmetry false
point_group_symmetry False

$end

#2

You need either VCI or VPT2
https://manual.q-chem.com/latest/Ch10.S8.SS5.html

#3

Thank you for your quick reply.
I run the input file shown below (,which include VCI = 2). But the output file did not include the overtones and the combination tones. I think the calculation should show overtones in near-IR region, because polyethylene experimentally shows in the region. Is the input I run file wrong?

$molecule
0 1
C -4.3555400 0.6876800 0.0030300
C -3.4738600 -0.5508400 -0.0085800
H -4.1530300 1.3221800 -0.8657400
H -4.1873200 1.2799600 0.9081500
H -5.4116000 0.4014400 -0.0239900
C -1.9916500 -0.1828600 0.0125000
H -3.6972700 -1.1432100 -0.9035200
H -3.7164100 -1.1730100 0.8608500
C -1.1101600 -1.4215700 0.0168400
H -1.7692300 0.4199200 0.9006700
H -1.7479400 0.4290500 -0.8639100
H -1.3002400 -2.0346600 0.9036400
H -0.0538300 -1.1350800 0.0222500
H -1.2912400 -2.0356600 -0.8710600
$end

$rem
METHOD hf
BASIS sto-3g

JOBTYPE opt

$end

@@@

$molecule
read
$end

$rem
METHOD hf
BASIS sto-3g

JOBTYPE freq
ANHAR TRUE
VCI 2
MODE_COUPLING 3

$end

#4

I don’t really know anything about this, but I experimented a bit with CO2, which as a well-known Fermi resonance, and it doesn’t seem like anything is available (even at high orders of VCI with n-mode coupling) beyond anharmonic corrections to the fundamentals.

Someone who is more familiar with this method is welcome to correct me…

1 Like
#5

I contacted Q-Chem support about overtone and combination bands.

I got good comment as shown below.
"we can inform you that Q-Chem’s anharmonic-correction feature is currently limited to fundamental vibrational modes only. We will have a look and create a feature request and get back as soon as the feature gets developed or if we have any related updates. "

I look forward to hearing the development about overtone and combination bands.