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fix colvars command

Syntax

fix ID group-ID colvars *configfile* keyword value ...

ID, group-ID are documented in fix command
“colvars” = style name of this fix command
configfile = configuration file for Colvars (use “none” to provide it inline)

keyword = output or input or unwrap or tstat or seed

output value = state filename/prefix for Colvars (default: "out")
input value = input state filename/prefix for Colvars (optional, default: "NULL")
unwrap value = "yes" or "no" (default: "yes")
tstat value = fix ID of thermostat applied to relevant atoms (default: "NULL")
seed value = seed for random number generator (default: 1966)

Examples

# Create the fix using a config file, set prefix for its output files
fix Colvars all colvars colvars.inp output ${JOB}

# Communicate the LAMMPS target temperature to the Colvars module
fix_modify Colvars tstat NPT

# Add a new restraint specific to this LAMMPS run
fix_modify Colvars config """
harmonic {
    name restraint
    colvars distance1 distance2
    centers ${ref1} ${ref2}
    forceConstant ${kappa}
}"""

Description

This fix interfaces LAMMPS to the collective variables Colvars library, which allows to accelerate sampling of rare events and the computation of free energy surfaces and potentials of mean force (PMFs) for any set of collective variables using a variety of sampling methods (e.g. umbrella-sampling, metadynamics, ABF…).

This documentation describes only the “fix colvars” command itself in a LAMMPS script. The Colvars library is fully documented in the included PDF manual or in the webpage https://colvars.github.io/colvars-refman-lammps/colvars-refman-lammps.html.

The Colvars library is developed at https://github.com/Colvars/colvars. A detailed discussion of its implementation is in (Fiorin); additional citations for specific features are printed at runtime if these features are used.

There are example scripts on the Colvars website as well as in the examples/PACKAGES/colvars directory in the LAMMPS source tree.

The only required argument to the fix is the name of the Colvars configuration file. The contents of this file are independent from the MD engine in which the Colvars library has been integrated, save for the units that are specific to each engine. In LAMMPS, the units used by Colvars are consistent with those specified by the units command.

Added in version Colvars_2023-06-04: The special value “none” (lowercase) initializes an empty Colvars module, which allows loading configuration dynamically using fix_modify (see below).

The group-ID entry is ignored. “fix colvars” will always apply to the entire system, but specific atoms will be selected based on selection keywords in the Colvars configuration file or files. There is no need to define multiple “fix colvars” instances and it is not allowed.

The “output” keyword allows to specify the prefix of output files generated by Colvars, for example “output.colvars.traj” or “output.pmf”. Supplying an empty string suppresses any file output from Colvars to file, except for data saved into the LAMMPS binary restart files.

The “input” keyword allows to specify an optional state file that contains the restart information needed to continue a previous simulation state. However, because “fix colvars” records its state in LAMMPS binary restart files, this is usually not needed when using the read_restart command.

The unwrap keyword controls whether wrapped or unwrapped coordinates are passed to the Colvars library for calculation of the collective variables and the resulting forces. The default is yes, i.e. the image flags are used to reconstruct the absolute atom positions. Setting this to no will use the current local coordinates that are wrapped back into the simulation cell at each re-neighboring step instead. For information about when and how this affects results, please see https://colvars.github.io/colvars-refman-lammps/colvars-refman-lammps.html#sec:colvar_atom_groups_wrapping.

The tstat keyword can be either “NULL” or the label of a thermostatting fix that thermostats all atoms in the fix colvars group. This will be used to provide the colvars module with the current thermostat target temperature.

The seed keyword contains the seed for the random number generator that will be used in the colvars module.

Restarting

This fix writes the current state of the Colvars module into binary restart files. This is in addition to the text-mode “.colvars.state” state file that is written by the Colvars module itself. The information contained in both files is identical, and the binary LAMMPS restart file is also used by fix colvars when read_restart is called in a LAMMPS script. In that case, there is typically no need to specify the input keyword.

As long as LAMMPS binary restarts will be used to continue a simulation, it is safe to delete the “.colvars.state” files to save space. However, when a LAMMPS simulation is restarted using read_data, the Colvars state file must be available and loaded via the “input” keyword or via a “fix_modify Colvars load” command (see below).

When restarting, the fix and the Colvars module should be created and configured using the original configuration file(s).

Output

This fix computes a global scalar which can be accessed by various output commands. The scalar is the energy due to all external potentials defined in the Colvars configuration. The scalar value calculated by this fix is “extensive”.

Aside from the state information in a “.colvars.state” file, other output files are produced by Colvars depending on the type of simulation. For this reason, the “output” keyword is required for fix colvars.

Controlling Colvars via fix_modify

Added in version Colvars_2023-06-04.

The fix_modify command may be used on “fix colvars” in either one of two ways:

Provide updated values for the fix parameters, such as output, input, unwrap, tstat and seed. Additionally, the fix_modify energy keyword is supported by this fix to add the energy change from the biasing force added by Colvars to the global potential energy of the system as part of thermodynamic output (the default is fix_modify energy no). For example, in a multi-step LAMMPS script involving multiple thermostats (e.g. fix nvt followed by fix npt), Colvars can read a new thermostat’s target temperature like this:

fix NVT all nvt ...
fix Colvars all colvars <configfile> output equil1 tstat NVT
run <NUMSTEPS>
unfix nvt
fix NPT all n ...
fix_modify Colvars tstat NPT
fix_modify Colvars output equil2

Call one of the scripting functions provided by the Colvars module itself (a full list is available in the Colvars doc). The arguments to these functions are provided as strings and passed to Colvars.

LAMMPS variables referenced by their string representation “${variable}” will be expanded immediately. Note also that this variable expansion will also happen within quotes, similar to what the mdi command provides. This feature makes it possible to use the values of certain LAMMPS variables in Colvars configuration strings. For example, to synchronize the LAMMPS and Colvars dump frequencies:

variable freq index 10000
dump myDump all atom/zstd ${freq} dump.atom.zstd
fix_modify Colvars config "colvarsTrajFrequency ${freq}"

Note

Although it is possible to use fix_modify at any time, its results will only reflect the state of the Colvars module at the end of the most recent “run” or “minimize” command. Any new configuration added via “fix_modify Colvars configfile” or “fix_modify Colvars config” will only be loaded when the simulation resumes. Configuration files or strings will be parsed in the same sequence as they were provided in the LAMMPS script.

Restrictions

This fix is provided by the COLVARS package and is only available if LAMMPS was built with that package (default in most builds). Some of the features also require code available from the LEPTON package. See the Build package page for more info.

There can only be one Colvars instance defined at a time. Since the interface communicates only the minimum required amount of information, and the Colvars module itself can handle an arbitrary number of collective variables, this is not a limitation of functionality.