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Dihedral Angle Restraints

The following example shows how to set up torsion angle restraints. Coupling constant measurements provide a means to obtain information about certain dihedral angles, albeit with low precision. The low precision makes it necessary to employ rather large error ranges.

restraints 
  dihedral

    reset

    assign (resid 1 and name c) (resid 2 and name ca)
      (resid 2 and name n) (resid 2 and name c)  1 -120.0 40.0 2   {* 9 Hz *}
    assign (resid 2 and name c) (resid 3 and name ca)
      (resid 3 and name n) (resid 3 and name c)  1 -120.0 50.0 2   {* 8 Hz *}

end

The four selections of each assign statement specify the particular dihedral angle. The first number after the selections specifies the energy constant (

) in kcal mole $^{-1}$ rad $^{-2}$ , the second number specifies degrees to which the dihedral angle is restrained ( ${\phi}_o$ ), the third number specifies the range around the restrained angle ( $\Delta \phi$ ), and the last number specifies the exponent of the restraining function. (Refer to Section 7.2 for more information.)

Xplor-NIH 2025-11-07