11.11 Anharmonic Vibrational Frequencies

11.11.4 Job Control

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(NVib+NVCI)/NVibNVCI 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×10-5.
RECOMMENDATION:
       Use the default, unless the potential surface is very flat, in which case a larger value should be used.

Example 11.17  A four-quanta anharmonic frequency calculation on formaldehyde at the EDF2/6-31G* optimized ground state geometry, which is obtained in the first part of the job. It is necessary to carry out the harmonic frequency first and this will print out an approximate time for the subsequent anharmonic frequency calculation. If a FREQ job has already been performed, the anharmonic calculation can be restarted using the saved scratch files from the harmonic calculation.

$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 11.10.3).

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

Example 11.18  This example shows an anharmonic frequency calculation for ethene where only the C-H stretching modes are included in the anharmonic analysis.

$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