102 lines
2.9 KiB
Plaintext
102 lines
2.9 KiB
Plaintext
.. index:: dihedral_style spherical
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dihedral_style spherical command
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================================
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Syntax
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""""""
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.. parsed-literal::
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dihedral_style spherical
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Examples
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""""""""
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.. parsed-literal::
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dihedral_coeff 1 1 286.1 1 124 1 1 90.0 0 1 90.0 0
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dihedral_coeff 1 3 286.1 1 114 1 1 90 0 1 90.0 0 &
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17.3 0 0.0 0 1 158 1 0 0.0 0 &
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15.1 0 0.0 0 0 0.0 0 1 167.3 1
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Description
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"""""""""""
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The *spherical* dihedral style uses the potential:
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.. image:: JPG/dihedral_spherical_angles.jpg
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:align: center
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.. image:: Eqs/dihedral_spherical.jpg
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:align: center
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For this dihedral style, the energy can be any function that combines the
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4-body dihedral-angle (phi) and the two 3-body bond-angles (theta1, theta2).
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For this reason, there is usually no need to define 3-body "angle" forces
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separately for the atoms participating in these interactions.
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It is probably more efficient to incorporate 3-body angle forces into
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the dihedral interaction even if it requires adding additional terms to
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the expansion (as was done in the second example). A careful choice of
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parameters can prevent singularities that occur with traditional
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force-fields whenever theta1 or theta2 approach 0 or 180 degrees.
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The last example above corresponds to an interaction with a single energy
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minima located at phi=114, theta1=158, theta2=167.3 degrees, and it remains
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numerically stable at all angles (phi, theta1, theta2). In this example,
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the coefficients 17.3, and 15.1 can be physically interpreted as the
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harmonic spring constants for theta1 and theta2 around their minima.
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The coefficient 286.1 is the harmonic spring constant for phi after
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division by sin(158)*sin(167.3) (the minima positions for theta1 and theta2).
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The following coefficients must be defined for each dihedral type via the
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:doc:`dihedral_coeff <dihedral_coeff>` command as in the example above, or in
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the Dihedral Coeffs section of a data file file read by the
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:doc:`read_data <read_data>` command:
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* n (integer >= 1)
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* C1 (energy)
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* K1 (typically an integer)
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* a1 (degrees)
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* u1 (typically 0.0 or 1.0)
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* L1 (typically an integer)
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* b1 (degrees, typically 0.0 or 90.0)
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* v1 (typically 0.0 or 1.0)
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* M1 (typically an integer)
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* c1 (degrees, typically 0.0 or 90.0)
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* w1 (typically 0.0 or 1.0)
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* ....
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* Cn (energy)
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* Kn (typically an integer)
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* an (degrees)
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* un (typically 0.0 or 1.0)
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* Ln (typically an integer)
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* bn (degrees, typically 0.0 or 90.0)
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* vn (typically 0.0 or 1.0)
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* Mn (typically an integer)
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* cn (degrees, typically 0.0 or 90.0)
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* wn (typically 0.0 or 1.0)
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----------
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Restrictions
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""""""""""""
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This dihedral style can only be used if LAMMPS was built with the
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USER_MISC package. See the :ref:`Making LAMMPS <start_3>`
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section for more info on packages.
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Related commands
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""""""""""""""""
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:doc:`dihedral_coeff <dihedral_coeff>`
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**Default:** none
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.. _lws: http://lammps.sandia.gov
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.. _ld: Manual.html
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.. _lc: Section_commands.html#comm
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