175 lines
6.3 KiB
ReStructuredText
175 lines
6.3 KiB
ReStructuredText
.. index:: fix cmap
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fix cmap command
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================
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Syntax
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""""""
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.. code-block:: LAMMPS
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fix ID group-ID cmap filename
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* ID, group-ID are documented in :doc:`fix <fix>` command
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* cmap = style name of this fix command
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* filename = force-field file with CMAP coefficients
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Examples
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""""""""
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.. code-block:: LAMMPS
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fix myCMAP all cmap ../potentials/cmap36.data
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read_data proteinX.data fix myCMAP crossterm CMAP
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fix_modify myCMAP energy yes
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Description
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"""""""""""
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This command enables CMAP 5-body interactions to be added to
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simulations which use the CHARMM force field. These are relevant for
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any CHARMM model of a peptide or protein sequences that is 3 or more
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amino-acid residues long; see :ref:`(Buck) <Buck>` and :ref:`(Brooks)
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<Brooks2>` for details, including the analytic energy expressions for
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CMAP interactions. The CMAP 5-body terms add additional potential
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energy contributions to pairs of overlapping phi-psi dihedrals of
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amino-acids, which are important to properly represent their
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conformational behavior.
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The examples/cmap directory has a sample input script and data file
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for a small peptide, that illustrates use of the fix cmap command.
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As in the example above, this fix should be used before reading a data
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file that contains a listing of CMAP interactions. The *filename*
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specified should contain the CMAP parameters for a particular version
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of the CHARMM force field. Two such files are including in the
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lammps/potentials directory: charmm22.cmap and charmm36.cmap.
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The data file read by the "read_data" must contain the topology of all
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the CMAP interactions, similar to the topology data for bonds, angles,
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dihedrals, etc. Specially it should have a line like this in its
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header section:
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.. parsed-literal::
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N crossterms
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where N is the number of CMAP 5-body interactions. It should also
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have a section in the body of the data file like this with N lines:
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.. parsed-literal::
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CMAP
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1 1 8 10 12 18 20
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2 5 18 20 22 25 27
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[...]
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N 3 314 315 317 318 330
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The first column is an index from 1 to N to enumerate the CMAP 5-atom
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tuples; it is ignored by LAMMPS. The second column is the "type" of
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the interaction; it is an index into the CMAP force field file. The
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remaining 5 columns are the atom IDs of the atoms in the two 4-atom
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dihedrals that overlap to create the CMAP interaction. Note that the
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"crossterm" and "CMAP" keywords for the header and body sections match
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those specified in the read_data command following the data file name;
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see the :doc:`read_data <read_data>` page for more details.
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A data file containing CMAP 5-body interactions can be generated from
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a PDB file using the charmm2lammps.pl script in the tools/ch2lmp
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directory of the LAMMPS distribution. The script must be invoked with
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the optional "-cmap" flag to do this; see the tools/ch2lmp/README file
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for more information. The same conversion script also creates the
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file of CMAP coefficient data which is read by this command.
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The potential energy associated with CMAP interactions can be output
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as described below. It can also be included in the total potential
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energy of the system, as output by the :doc:`thermo_style
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<thermo_style>` command, if the :doc:`fix_modify energy <fix_modify>`
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command is used, as in the example above. See the note below about
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how to include the CMAP energy when performing an :doc:`energy
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minimization <minimize>`.
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----------
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Restart, fix_modify, output, run start/stop, minimize info
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"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
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This fix writes the list of CMAP cross-terms to :doc:`binary restart
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files <restart>`. See the :doc:`read_restart <read_restart>` command
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for info on how to re-specify a fix in an input script that reads a
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restart file, so that the operation of the fix continues in an
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uninterrupted fashion.
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The :doc:`fix_modify <fix_modify>` *energy* option is supported by
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this fix to add the potential energy of the CMAP interactions to both
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the global potential energy and peratom potential energies of the
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system as part of :doc:`thermodynamic output <thermo_style>` or
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output by the :doc:`compute pe/atom <compute_pe_atom>` command. The
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default setting for this fix is :doc:`fix_modify energy yes
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<fix_modify>`.
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The :doc:`fix_modify <fix_modify>` *virial* option is supported by
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this fix to add the contribution due to the CMAP interactions to both
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the global pressure and per-atom stress of the system via the
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:doc:`compute pressure <compute_pressure>` and :doc:`compute
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stress/atom <compute_stress_atom>` commands. The former can be
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accessed by :doc:`thermodynamic output <thermo_style>`. The default
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setting for this fix is :doc:`fix_modify virial yes <fix_modify>`.
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This fix computes a global scalar which can be accessed by various
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:doc:`output commands <Howto_output>`. The scalar is the potential
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energy discussed above. The scalar value calculated by this fix is
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"extensive".
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No parameter of this fix can be used with the *start/stop* keywords of
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the :doc:`run <run>` command.
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The forces due to this fix are imposed during an energy minimization,
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invoked by the :doc:`minimize <minimize>` command.
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The :doc:`fix_modify <fix_modify>` *respa* option is supported by this
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fix. This allows to set at which level of the :doc:`r-RESPA
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<run_style>` integrator the fix is adding its forces. Default is the
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outermost level.
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.. note::
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For energy minimization, if you want the potential energy
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associated with the CMAP terms forces to be included in the total
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potential energy of the system (the quantity being minimized), you
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MUST not disable the :doc:`fix_modify <fix_modify>` *energy* option
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for this fix.
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Restrictions
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""""""""""""
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To function as expected this fix command must be issued *before* a
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:doc:`read_data <read_data>` command but *after* a
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:doc:`read_restart <read_restart>` command.
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This fix can only be used if LAMMPS was built with the MOLECULE
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package. See the :doc:`Build package <Build_package>` page for more
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info.
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Related commands
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""""""""""""""""
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:doc:`fix_modify <fix_modify>`, :doc:`read_data <read_data>`
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Default
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"""""""
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none
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----------
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.. _Buck:
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**(Buck)** Buck, Bouguet-Bonnet, Pastor, MacKerell Jr., Biophys J, 90, L36
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(2006).
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.. _Brooks2:
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**(Brooks)** Brooks, Brooks, MacKerell Jr., J Comput Chem, 30, 1545 (2009).
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