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@ -46,25 +46,26 @@ Description
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"""""""""""
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This fix interfaces LAMMPS to the collective variables `Colvars
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<https://colvars.github.io>`_ library, which allows to accelerate sampling of
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rare events and the computation of free energy surfaces and potentials of
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mean force (PMFs) for any set of collective variables using a variety of
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sampling methods (e.g. umbrella-sampling, metadynamics, ABF...).
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<https://colvars.github.io>`_ library, which allows to accelerate
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sampling of rare events and the computation of freeenergy surfaces and
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potentials of mean force (PMFs) for any set of collective variables
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using a variety of sampling methods (e.g. umbrella-sampling,
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metadynamics, ABF...).
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This documentation describes only the "fix colvars" command itself in
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a LAMMPS script. The Colvars library is fully documented in the included
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This documentation describes only the "fix colvars" command itself in a
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LAMMPS script. The Colvars library is fully documented in the included
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`PDF manual <PDF/colvars-refman-lammps.pdf>`_ or in the webpage
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`https://colvars.github.io/colvars-refman-lammps/colvars-refman-lammps.html
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<https://colvars.github.io/colvars-refman-lammps/colvars-refman-lammps.html>`_.
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The Colvars library is developed at `https://github.com/Colvars/colvars
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<https://github.com/colvars/colvars>`_ A detailed discussion of its
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<https://github.com/colvars/colvars>`_. A detailed discussion of its
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implementation is in :ref:`(Fiorin) <Fiorin>`; additional citations for
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specific features are printed at runtime if these features are used.
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There are example scripts on the `Colvars website <https://colvars.github.io>`_
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as well as in the ``examples/PACKAGES/colvars`` directory in the LAMMPS
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source tree.
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There are example scripts on the `Colvars website
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<https://colvars.github.io>`_ as well as in the
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``examples/PACKAGES/colvars`` directory in the LAMMPS source tree.
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----------
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@ -75,42 +76,44 @@ units that are specific to each engine. In LAMMPS, the units used by
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Colvars are consistent with those specified by the :doc:`units <units>`
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command.
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.. versionadded:: Colvars_2023-06-04 The special value "*none*"
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(lowercase) initializes an empty Colvars module, which
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allows loading configuration dynamically using
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:doc:`fix_modify <fix_modify>` (see below).
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.. versionadded:: Colvars_2023-06-04
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The *group-ID* entry is ignored. "fix colvars" will always apply to
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the entire system, but specific atoms will be selected based on
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selection keywords in the Colvars configuration file or files. There is
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no need to define multiple "fix colvars" instances and it is not
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allowed.
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The special value "*none*" (lowercase) initializes an empty Colvars
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module, which allows loading configuration dynamically using
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:doc:`fix_modify <fix_modify>` (see below).
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The "output" keyword allows to specify the prefix of output files generated
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by Colvars, for example "*output*.colvars.traj" or "output.pmf". Supplying
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an empty string suppresses any file output from Colvars to file, except for
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data saved into the LAMMPS :doc:`binary restart <restart>` files.
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The *group-ID* entry is ignored. "fix colvars" will always apply to the
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entire system, but specific atoms will be selected based on selection
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keywords in the Colvars configuration file or files. There is no need
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to define multiple "fix colvars" instances and it is not allowed.
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The "input" keyword allows to specify an optional state file that contains
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the restart information needed to continue a previous simulation state.
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However, because "fix colvars" records its state in LAMMPS :doc:`binary
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restart <restart>` files, this is usually not needed when using the
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:doc:`read_restart <read_restart>` command.
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The "output" keyword allows to specify the prefix of output files
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generated by Colvars, for example "*output*.colvars.traj" or
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"output.pmf". Supplying an empty string suppresses any file output from
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Colvars to file, except for data saved into the LAMMPS :doc:`binary
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restart <restart>` files.
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The *unwrap* keyword controls whether wrapped or unwrapped coordinates are
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passed to the Colvars library for calculation of the collective variables and
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the resulting forces. The default is *yes*, i.e. the image flags are used to
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reconstruct the absolute atom positions. Setting this to *no* will use the
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current local coordinates that are wrapped back into the simulation cell at
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each re-neighboring step instead. For information about when and how this
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affects results, please see
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The "input" keyword allows to specify an optional state file that
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contains the restart information needed to continue a previous
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simulation state. However, because "fix colvars" records its state in
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LAMMPS :doc:`binary restart <restart>` files, this is usually not needed
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when using the :doc:`read_restart <read_restart>` command.
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The *unwrap* keyword controls whether wrapped or unwrapped coordinates
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are passed to the Colvars library for calculation of the collective
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variables and the resulting forces. The default is *yes*, i.e. the
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image flags are used to reconstruct the absolute atom positions.
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Setting this to *no* will use the current local coordinates that are
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wrapped back into the simulation cell at each re-neighboring step
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instead. For information about when and how this affects results,
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please see
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`https://colvars.github.io/colvars-refman-lammps/colvars-refman-lammps.html#sec:colvar_atom_groups_wrapping
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<https://colvars.github.io/colvars-refman-lammps/colvars-refman-lammps.html#sec:colvar_atom_groups_wrapping>`_.
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The *tstat* keyword can be either "NULL" or the label of a thermostatting
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fix that thermostats all atoms in the fix colvars group. This will be
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used to provide the colvars module with the current thermostat target
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temperature.
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The *tstat* keyword can be either "NULL" or the label of a
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thermostatting fix that thermostats all atoms in the fix colvars
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group. This will be used to provide the colvars module with the current
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thermostat target temperature.
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The *seed* keyword contains the seed for the random number generator
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that will be used in the colvars module.
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@ -119,36 +122,38 @@ that will be used in the colvars module.
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Restarting
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""""""""""
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This fix writes the current state of the Colvars module into :doc:`binary
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restart files <restart>`. This is in addition to the text-mode
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".colvars.state" state file that is written by the Colvars module itself.
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The information contained in both files is identical, and the binary LAMMPS
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restart file is also used by fix colvars when :doc:`read_restart
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<read_restart>` is called in a LAMMPS script. In that case, there is
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typically no need to specify the *input* keyword.
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This fix writes the current state of the Colvars module into
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:doc:`binary restart files <restart>`. This is in addition to the
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text-mode ".colvars.state" state file that is written by the Colvars
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module itself. The information contained in both files is identical,
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and the binary LAMMPS restart file is also used by fix colvars when
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:doc:`read_restart <read_restart>` is called in a LAMMPS script. In
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that case, there is typically no need to specify the *input* keyword.
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As long as LAMMPS binary restarts will be used to continue a simulation, it
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is safe to delete the ".colvars.state" files to save space. However, when a
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LAMMPS simulation is restarted using :doc:`read_data <read_data>`, the
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Colvars state file must be available and loaded via the "input" keyword or
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via a "fix_modify Colvars load" command (see below).
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As long as LAMMPS binary restarts will be used to continue a simulation,
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it is safe to delete the ".colvars.state" files to save space. However,
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when a LAMMPS simulation is restarted using :doc:`read_data
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<read_data>`, the Colvars state file must be available and loaded via
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the "input" keyword or via a "fix_modify Colvars load" command (see
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below).
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When restarting, the fix and the Colvars module should be created and
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configured using either the original configuration file(s).
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configured using the original configuration file(s).
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Output
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""""""
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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 energy due to all
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external potentials defined in the Colvars configuration. The scalar value
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calculated by this fix is "extensive".
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:doc:`output commands <Howto_output>`. The scalar is the energy due to
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all external potentials defined in the Colvars configuration. The
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scalar value calculated by this fix is "extensive".
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Aside from the state information in a ".colvars.state" file, other
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`output files <https://colvars.github.io/colvars-refman-lammps/colvars-refman-lammps.html#sec:colvars_output>`_
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are produced by Colvars depending on the type of simulation.
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For this reason, the "output" keyword is required for fix colvars.
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`output files
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<https://colvars.github.io/colvars-refman-lammps/colvars-refman-lammps.html#sec:colvars_output>`_
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are produced by Colvars depending on the type of simulation. For this
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reason, the "output" keyword is required for fix colvars.
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Controlling Colvars via `fix_modify`
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@ -156,18 +161,18 @@ Controlling Colvars via `fix_modify`
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.. versionadded:: Colvars_2023-06-04
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The :doc:`fix_modify <fix_modify>` command may be used on "fix colvars" in
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either one of two ways:
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The :doc:`fix_modify <fix_modify>` command may be used on "fix colvars"
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in either one of two ways:
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(1) Provide updated values for the fix parameters, such as *output*, *input*,
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*unwrap*, *tstat* and *seed*. Additionally, the :doc:`fix_modify
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<fix_modify>` *energy* keyword is supported by this fix to add the energy
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change from the biasing force added by Colvars to the global potential
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energy of the system as part of :doc:`thermodynamic output <thermo_style>`
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(the default is :doc:`fix_modify energy no <fix_modify>`).
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For example, in a multi-step LAMMPS script involving multiple thermostats
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(e.g. fix nvt followed by fix npt), Colvars can read a new thermostat's
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target temperature like this:
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(1) Provide updated values for the fix parameters, such as *output*,
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*input*, *unwrap*, *tstat* and *seed*. Additionally, the
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:doc:`fix_modify <fix_modify>` *energy* keyword is supported by this fix
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to add the energy change from the biasing force added by Colvars to the
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global potential energy of the system as part of :doc:`thermodynamic
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output <thermo_style>` (the default is :doc:`fix_modify energy no
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<fix_modify>`). For example, in a multi-step LAMMPS script involving
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multiple thermostats (e.g. fix nvt followed by fix npt), Colvars can
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read a new thermostat's target temperature like this:
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.. code-block:: LAMMPS
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@ -186,10 +191,11 @@ either one of two ways:
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LAMMPS variables referenced by their string representation
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"${variable}" will be expanded immediately. Note also that this
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variable expansion *will also happen within quotes*, similar to what the
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:doc:`mdi <mdi>` command provides. This feature makes it possible to use
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the values of certain LAMMPS variables in Colvars configuration strings.
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For example, to synchronize the LAMMPS and Colvars dump frequencies:
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variable expansion *will also happen within quotes*, similar to what
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the :doc:`mdi <mdi>` command provides. This feature makes it
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possible to use the values of certain LAMMPS variables in Colvars
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configuration strings. For example, to synchronize the LAMMPS and
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Colvars dump frequencies:
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.. code-block:: LAMMPS
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@ -199,27 +205,27 @@ either one of two ways:
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.. note::
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Although it is possible to use :doc:`fix_modify <fix_modify>` at any time,
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its results will only reflect the state of the Colvars module at the end
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of the most recent "run" or "minimize" command. Any new configuration
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added via "fix_modify Colvars configfile" or "fix_modify Colvars config"
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will only be loaded when the simulation resumes. Configuration files or
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strings will be parsed in the same sequence as they were provided in the
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LAMMPS script.
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Although it is possible to use :doc:`fix_modify <fix_modify>` at any
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time, its results will only reflect the state of the Colvars module
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at the end of the most recent "run" or "minimize" command. Any new
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configuration added via "fix_modify Colvars configfile" or
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"fix_modify Colvars config" will only be loaded when the simulation
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resumes. Configuration files or strings will be parsed in the same
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sequence as they were provided in the LAMMPS script.
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Restrictions
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""""""""""""
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This fix is provided by the COLVARS package and is only available if LAMMPS
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was built with that package (default in most builds). Some of the features
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also require code available from the LEPTON package. See the :doc:`Build
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package <Build_package>` page for more info.
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This fix is provided by the COLVARS package and is only available if
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LAMMPS was built with that package (default in most builds). Some of
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the features also require code available from the LEPTON package. See
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the :doc:`Build package <Build_package>` page for more info.
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There can only be one Colvars instance defined at a time. Since the
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interface communicates only the minimum required amount of information, and
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the Colvars module itself can handle an arbitrary number of collective
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variables, this is not a limitation of functionality.
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interface communicates only the minimum required amount of information,
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and the Colvars module itself can handle an arbitrary number of
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collective variables, this is not a limitation of functionality.
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Related commands
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Axel Kohlmeyer