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Properties

ROT

ROT — Defines rotational symmetry.

Usage

ROT=n

Description

In calculation of rotational contributions to thermodynamic quantities, the rotational symmetry number of the molecule can be supplied by using this keyword. This requires the calculation to properly account for rotational symmetry, leading to a more accurate result. The symmetry number of a point group is the number of equivalent positions attainable by pure rotations. No reflections or improper rotations are allowed. This number is assumed by default to be 1. The proper value may be affected by subtle modifications to the molecule, such as isotopic substitution. A list of the most important symmetry numbers as they relate to the point groups follows.

Table A.1. Symmetry Numbers

C1 Ci Cs   1     D2 D2d D2h   4     C∞v     1
C2 C2v C2h   2     D3 D3d D3h   6     D∞h     2
C3 C3v C3h   3     D4 D4d D4h   8     T Td   12
C4 C4v C4h   4     D6 D6d D6h   12     Oh     24
C6 C6v C6h   6     S6       3            

Abbreviation:

none

Requires:

none

Required by:

none

Default value:

none

See also:

FORCE, IRC, LTRD, NEWTON, THERMO

Examples:

  am1 rhf singlet t=auto truste force thermo rot=2 trans=2  1
p-xylene
Thermo with trans=2 to remove internal rotation of methyls
 C              0.000000  0    0.000000  0    0.000000  0    0    0    0  2
 C              1.395052  1    0.000000  0    0.000000  0    1    0    0
 C              1.396919  1  120.474038  1    0.000000  0    2    1    0
 C              1.396919  1  119.051836  1   -0.106881  1    3    2    1
 C              1.395052  1  120.474038  1    0.106881  1    4    3    2
 C              1.396919  1  120.474038  1    0.108409  1    1    2    3
 H              1.100167  1  119.542900  1  179.980911  1    1    6    5
 H              1.100167  1  119.983003  1 -179.979770  1    2    1    6
 H              1.100167  1  119.542900  1 -179.980911  1    4    3    2
 H              1.100167  1  119.983003  1  179.979770  1    5    4    3
 C              1.480664  1  120.472114  1 -179.388508  1    3    2    1
 H              1.119254  1  109.937276  1   89.635686  1   11    3    2
 H              1.116344  1  111.043245  1 -149.505323  1   11    3    2
 H              1.116344  1  111.043245  1  -31.223306  1   11    3    2
 C              1.480664  1  120.472114  1 -179.388508  1    6    1    2
 H              1.116344  1  111.043245  1 -149.505323  1   15    6    1
 H              1.116344  1  111.043245  1  -31.223306  1   15    6    1
 H              1.119254  1  109.937276  1   89.635686  1   15    6    1
 0              0.000000  0    0.000000  0    0.000000  0    0    0    0
1

The keyword FORCE calls for a frequency calculation (in Cartesians) along with the generation of thermodynamic data. THERMO can only be requested if a keyword requiring computation of the Hessian (matrix of second derivatives) is used (see discussion of the THERMO keyword). In order to obtain correct results for the energy partition, the symmetry number for the point group must be given. The TS's point group is C2h , requiring ROT=2. The keyword TRANS=2 is optional as the new AMPAC automatically accounts for the presence of two negative eigenvalues.
2

This geometry was pre-optimized. If the geometry is not pre-optimized, then one of AMPAC's geometry optimizers (energy minimizers: TRUSTE, EF, NEWTON, BFGS, DFP; gradient minimizers: TRUSTG, TS, or LTRD) should be specified along with FORCE, so that the geometry will be optimized just prior to computing frequencies.
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