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Practical methods for including torsional anharmonicity in thermochemical calculations on complex molecules: the

Jingjing Zheng1, Tao Yu, Ewa Papajak

  • 1Department of Chemistry and Supercomputing Institute, University of Minnesota, Minneapolis, MN 55455-0431, USA.

Physical Chemistry Chemical Physics : PCCP
|May 13, 2011
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New multi-structural (MS) methods improve partition function calculations by using internal coordinates to correct for torsional motions, avoiding problematic normal mode assignments. These methods offer a more robust approach to accounting for complex molecular dynamics.

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Area of Science:

  • Computational Chemistry
  • Molecular Modeling
  • Physical Chemistry

Background:

  • Harmonic partition function calculations often simplify torsional motions using one-dimensional treatments.
  • Torsional motions are frequently coupled to other molecular degrees of freedom, complicating their accurate harmonic treatment.
  • Assigning specific normal modes to coupled torsions is problematic and can lead to inaccuracies.

Purpose of the Study:

  • To develop a new class of methods, multi-structural (MS) methods, to accurately correct partition functions for torsional motions.
  • To circumvent the need for assigning torsions to specific normal modes.
  • To adjust harmonic results using torsional correction factors derived from internal coordinates.

Main Methods:

  • Introduced three multi-structural (MS) methods: MS-AS (all structures), MS-ASCB (all structures with conformational barriers), and MS-RS (reference structure).
  • MS-AS involves including all conformers generated by internal rotations.
  • MS-ASCB includes explicit calculations of conformational barrier heights.
  • MS-RS considers conformers generated from a reference structure by independent torsions.

Main Results:

  • Demonstrated the application of MS methods to ethanol, 1-butanol, and 1-pentyl radical.
  • Illustrated the methods with calculations on two one-dimensional torsional potentials.
  • The MS-AS method showed promise by not requiring conformational barrier information or connectivity paths.

Conclusions:

  • Multi-structural (MS) methods provide a robust alternative for correcting harmonic partition functions for torsional motions.
  • These methods effectively handle strong couplings between torsions and other molecular vibrations.
  • The MS-AS method offers a simplified approach by eliminating the need for explicit barrier and path data.