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The following *$rem* variables can be used to control the calculation of
anharmonic frequencies.

ANHAR

Performing various nuclear vibrational theory (TOSH, VPT2, VCI) calculations
to obtain vibrational anharmonic frequencies.

TYPE:

LOGICAL

DEFAULT:

FALSE

OPTIONS:

TRUE
Carry out the anharmonic frequency calculation.
FALSE
Do harmonic frequency calculation.

RECOMMENDATION:

Since this calculation involves the third and fourth derivatives at the
minimum of the potential energy surface, it is recommended that the
GEOM_OPT_TOL_DISPLACEMENT, GEOM_OPT_TOL_GRADIENT and
GEOM_OPT_TOL_ENERGY tolerances are set tighter. Note that VPT2
calculations may fail if the system involves accidental degenerate resonances.
See the VCI *$rem* variable for more details about increasing the
accuracy of anharmonic calculations.

VCI

Specifies the number of quanta involved in the VCI calculation.

TYPE:

INTEGER

DEFAULT:

0

OPTIONS:

User-defined. Maximum value is 10.

RECOMMENDATION:

The availability depends on the memory of the machine. Memory allocation for
VCI calculation is the square of
$2({N}_{\mathrm{Vib}}+{N}_{\mathrm{VCI}})/{N}_{\mathrm{Vib}}{N}_{\mathrm{VCI}}$ with double
precision. For example, a machine with 1.5 GB memory and for molecules with
fewer than 4 atoms, VCI(10) can be carried out, for molecule containing fewer
than 5 atoms, VCI(6) can be carried out, for molecule containing fewer than 6
atoms, VCI(5) can be carried out. For molecules containing fewer than 50
atoms, VCI(2) is available. VCI(1) and VCI(3) usually overestimated the true
energy while VCI(4) usually gives an answer close to the converged energy.

FDIFF_DER

Controls what types of information are used to compute higher
derivatives. The default uses a combination of energy, gradient and Hessian
information, which makes the force field calculation faster.

TYPE:

INTEGER

DEFAULT:

3
for jobs where analytical 2nd derivatives are available.
0
for jobs with ECP.

OPTIONS:

0
Use energy information only.
1
Use gradient information only.
2
Use Hessian information only.
3
Use energy, gradient, and Hessian information.

RECOMMENDATION:

When the molecule is larger than benzene with small basis set,
FDIFF_DER = 2 may be faster. Note that FDIFF_DER will be set
lower if analytic derivatives of the requested order are not available. Please
refers to IDERIV.

MODE_COUPLING

Number of modes coupling in the third and fourth derivatives calculation.

TYPE:

INTEGER

DEFAULT:

2
for two modes coupling.

OPTIONS:

$n$
for $n$ modes coupling, Maximum value is 4.

RECOMMENDATION:

Use the default.

IGNORE_LOW_FREQ

Low frequencies that should be treated as rotation can be ignored during
anharmonic correction calculation.

TYPE:

INTEGER

DEFAULT:

300
Corresponding to 300 cm${}^{-1}$.

OPTIONS:

$n$
Any mode with harmonic frequency less than $n$ will be ignored.

RECOMMENDATION:

Use the default.

FDIFF_STEPSIZE_QFF

Displacement used for calculating third and fourth derivatives by finite difference.

TYPE:

INTEGER

DEFAULT:

5291
Corresponding to 0.1 bohr. For calculating third and fourth derivatives.

OPTIONS:

$n$
Use a step size of $n\times {10}^{-5}$.

RECOMMENDATION:

Use the default, unless the potential surface is very flat, in which case a larger value
should be used.

$molecule 0 1 C O, 1, CO H, 1, CH, 2, A H, 1, CH, 2, A, 3, D CO = 1.2 CH = 1.0 A = 120.0 D = 180.0 $end $rem JOBTYPE OPT METHOD EDF2 BASIS 6-31G* GEOM_OPT_TOL_DISPLACEMENT 1 GEOM_OPT_TOL_GRADIENT 1 GEOM_OPT_TOL_ENERGY 1 $end @@@ $molecule READ $end $rem JOBTYPE FREQ METHOD EDF2 BASIS 6-31G* ANHAR TRUE VCI 4 $end

Anharmonic frequencies can also be computed using the partial hessian approximation (see Section 10.9.4).

ANHAR_SEL

Select a subset of normal modes for subsequent anharmonic frequency analysis.

TYPE:

LOGICAL

DEFAULT:

FALSE
Use all normal modes

OPTIONS:

TRUE
Select subset of normal modes

RECOMMENDATION:

None

$comment ethene restricted anharmonic frequency analysis $end $molecule 0 1 C 0.6665 0.0000 0.0000 C -0.6665 0.0000 0.0000 H 1.2480 0.9304 0.0000 H -1.2480 -0.9304 0.0000 H -1.2480 0.9304 0.0000 H 1.2480 -0.9304 0.0000 $end $rem JOBTYPE freq METHOD hf BASIS sto-3g ANHAR_SEL TRUE N_SOL 4 $end $alist 9 10 11 12 $end