- General options for a collective variable
- Artificial boundary potentials (walls)
- Trajectory output
- Extended Lagrangian.
- Statistical analysis of collective variables

Defining collective variables and their properties

In the configuration file each colvar is defined by the keyword
`colvar`, followed by its configuration options within curly braces: `colvar { ... }`. One of these options is the name of a colvar component: for example, including `rmsd { ... }` defines the colvar as a RMSD function. *In most applications, only one component is used, and the component is equal to the colvar.*

The full list of colvar components can be found in Section 10.4, with the syntax to select atoms in Section 10.3. The following section lists several options to control the behavior of a single colvar, regardless of its type.

General options for a collective variable

Name of this colvar`name`**Context:**`colvar`**Acceptable Values:**string**Default Value:**```colvar`'' + numeric id**Description:**The name is an unique case-sensitive string which allows the colvar module to identify this colvar unambiguously; it is also used in the trajectory file to label to the columns corresponding to this colvar.Expected fluctuations amplitude, and resolution for grid-based methods`width`**Context:**`colvar`**Acceptable Values:**positive decimal**Default Value:**1.0**Description:**This number is a user-provided estimate of the fluctuation amplitude for the colvar. For example, it is recommended to set this number smaller than or equal to the standard deviation of the colvar during a very short simulation run. Biasing algorithms use this parameter for different purposes: harmonic restraints (10.5.3) use it to set the physical unit of the force constant, the histogram (10.5.6) and ABF biases (10.5.1) interpret it as the grid spacing in the direction of this variable, and metadynamics (10.5.2) uses it to set the width of newly added hills. This number is expressed in the same physical unit as the colvar value.Lower boundary of the colvar`lowerBoundary`**Context:**`colvar`**Acceptable Values:**decimal**Description:**Defines the lowest end of the interval of ``relevant'' values for the colvar. This number can be either a true physical boundary, or a user-defined number. Together with`upperBoundary`and`width`, it is used to define a grid of values along the colvar (not available for colvars based on`distanceDir`,`distanceVec`, and`orientation`). This option does not affect dynamics: to confine a colvar within a certain interval, the options`lowerWall`and`lowerWallConstant`should be used.Upper boundary of the colvar`upperBoundary`**Context:**`colvar`**Acceptable Values:**decimal**Description:**Similarly to`lowerBoundary`, defines the highest possible or allowed value.Whether the lower boundary is the physical lower limit`hardLowerBoundary`**Context:**`colvar`**Acceptable Values:**boolean**Default Value:**`off`**Description:**This option does not affect simulation results, but enables some internal optimizations. Depending on its mathematical definition, a colvar may have ``natural'' boundaries: for example, a`distance`colvar has a ``natural'' lower boundary at 0. Setting this option instructs the colvars module that the user-defined lower boundary is ``natural''. See Section 10.4 for the physical ranges of values of each component.Whether the upper boundary is the physical upper limit of the colvar's values`hardUpperBoundary`**Context:**`colvar`**Acceptable Values:**boolean**Default Value:**`off`**Description:**Analogous to`hardLowerBoundary`.Allow to expand the two boundaries if needed`expandBoundaries`**Context:**`colvar`**Acceptable Values:**boolean**Default Value:**`off`**Description:**If defined, biasing and analysis methods may keep their own copies of`lowerBoundary`and`upperBoundary`, and expand them to accommodate values that do not fit in the initial range. Currently, this option is used by the metadynamics bias (10.5.2) to keep all of its hills fully within the grid. This option cannot be used when the initial boundaries already span the full period of a periodic colvar.

The following options are useful to define restraints (confining potentials) for this colvar.
To apply moving restraints, or restraints to more than one colvar simultaneously, a more convenient option is to use the `harmonic` bias (10.5.3).

Lower wall force constant (kcal/mol)`lowerWallConstant`**Context:**`colvar`**Acceptable Values:**positive decimal**Description:**Defines the force constant for a confining restraint on the colvar, in the form of a ``half-harmonic'' potential. The potential starts at`lowerWall`if it is defined, or`lowerBoundary`otherwise. The energy unit of the constant is kcal/mol, while the spatial unit is that of the colvar.Position of the lower wall`lowerWall`**Context:**`colvar`**Acceptable Values:**decimal**Default Value:**`lowerBoundary`**Description:**Defines the value below which a confining restraint on the colvar is applied, in the form of a ``half-harmonic'' potential. Allows to use a different position of the wall than`lowerBoundary`.Upper wall force constant (kcal/mol)`upperWallConstant`**Context:**`colvar`**Acceptable Values:**positive decimal**Description:**Analogous to`lowerWallConstant`.Position of the upper wall`upperWall`**Context:**`colvar`**Acceptable Values:**decimal**Default Value:**`upperBoundary`**Description:**Analogous to`lowerWall`.

Output a trajectory for this colvar`outputValue`**Context:**`colvar`**Acceptable Values:**boolean**Default Value:**`on`**Description:**If`colvarsTrajFrequency`is non-zero, the value of this colvar is written to the trajectory file every`colvarsTrajFrequency`steps in the column labeled ```name`''.Output a velocity trajectory for this colvar`outputVelocity`**Context:**`colvar`**Acceptable Values:**boolean**Default Value:**`off`**Description:**If`colvarsTrajFrequency`is defined, the finite-difference calculated velocity of this colvar are written to the trajectory file under the label ```v_``name`''.Output an energy trajectory for this colvar`outputEnergy`**Context:**`colvar`**Acceptable Values:**boolean**Default Value:**`off`**Description:**This option applies only to extended Lagrangian colvars. If`colvarsTrajFrequency`is defined, the kinetic energy of the extended degree and freedom and the potential energy of the restraining spring are are written to the trajectory file under the labels ```Ek_``name`'' and ```Ep_``name`''.Output a system force trajectory for this colvar`outputSystemForce`**Context:**`colvar`**Acceptable Values:**boolean**Default Value:**`off`**Description:**If`colvarsTrajFrequency`is defined, the total system force on this colvar (i.e. the projection of all interatomic forces except constraint forces on this colvar -- see equation (53) in section 10.5.1) are written to the trajectory file under the label ```fs_``name`''. For extended Lagrangian colvars, the "system force" felt by the extended degree of freedom is simply the force from the harmonic spring.**Note:**not all components support this option. The physical unit for this force is kcal/mol, divided by the colvar unit.Output an applied force trajectory for this colvar`outputAppliedForce`**Context:**`colvar`**Acceptable Values:**boolean**Default Value:**`off`**Description:**If`colvarsTrajFrequency`is defined, the total force applied on this colvar by biases and confining potentials (walls) within the colvar module are written to the trajectory under the label ```fa_``name`''. For extended Lagrangian colvars, this force is actually applied to the extended degree of freedom rather than the geometric colvar itself. The physical unit for this force is kcal/mol divided by the colvar unit.

Extended Lagrangian.

The following options enable extended-system dynamics, where a colvar is coupled to an additional degree of freedom (fictitious particle) by a harmonic spring. All biasing and confining forces are then applied to the extended degree of freedom, and the actual, geometric colvar (function of Cartesian coordinates) only feels the force from the harmonic spring.

Add extended degree of freedom`extendedLagrangian`**Context:**`colvar`**Acceptable Values:**boolean**Default Value:**`off`**Description:**Adds a fictitious particle to be coupled to the colvar by a harmonic spring. The fictitious mass and the force constant of the coupling potential are derived from the parameters`extendedTimeConstant`and`extendedFluctuation`, described below. Biasing forces on the colvar are applied to this fictitious particle, rather than to the atoms directly. This implements the extended Lagrangian formalism used in some metadynamics simulations [36]. The energy associated with the extended degree of freedom is reported under the MISC title in NAMD's energy output.Standard deviation between the colvar and the fictitious particle (colvar unit)`extendedFluctuation`**Context:**`colvar`**Acceptable Values:**positive decimal**Description:**Defines the spring stiffness for the`extendedLagrangian`mode, by setting the typical deviation between the colvar and the extended degree of freedom due to thermal fluctuation. The spring force constant is calculated internally as , where is the value of`extendedFluctuation`.Oscillation period of the fictitious particle (fs)`extendedTimeConstant`**Context:**`colvar`**Acceptable Values:**positive decimal**Default Value:**`200`**Description:**Defines the inertial mass of the fictitious particle, by setting the oscillation period of the harmonic oscillator formed by the fictitious particle and the spring. The period should be much larger than the MD time step to ensure accurate integration of the extended particle's equation of motion. The fictitious mass is calculated internally as , where is the period and is the typical fluctuation (see above).Temperature for the extended degree of freedom (K)`extendedTemp`**Context:**`colvar`**Acceptable Values:**positive decimal**Default Value:**thermostat temperature**Description:**Temperature used for calculating the coupling force constant of the extended coordinate (see`extendedFluctuation`) and, if needed, as a target temperature for extended Langevin dynamics (see`extendedLangevinDamping`). This should normally be left at its default value.Damping factor for extended Langevin dynamics (ps )`extendedLangevinDamping`**Context:**`colvar`**Acceptable Values:**positive decimal**Default Value:**`1.0`**Description:**If this is non-zero, the extended degree of freedom undergoes Langevin dynamics at temperature`extendedTemp`. The friction force is minus`extendedLangevinDamping`times the velocity. This is useful because the extended dynamics coordinate may heat up in the transient non-equilibrium regime of ABF. Use moderate damping values, to limit viscous friction (potentially slowing down diffusive sampling) and stochastic noise (increasing the variance of statistical measurements). In doubt, use the default value.

Statistical analysis of collective variables

When the global keyword `analysis` is defined in the
configuration file, run-time calculations of statistical properties for
individual colvars can be performed. At the moment, several types of
time correlation functions, running averages and running standard
deviations are available.

Calculate a time correlation function?`corrFunc`**Context:**`colvar`**Acceptable Values:**boolean**Default Value:**`off`**Description:**Whether or not a time correlaction function should be calculated for this colvar.Colvar name for the correlation function`corrFuncWithColvar`**Context:**`colvar`**Acceptable Values:**string**Description:**By default, the auto-correlation function (ACF) of this colvar, , is calculated. When this option is specified, the correlation function is calculated instead with another colvar, , which must be of the same type (scalar, vector, or quaternion) as .Type of the correlation function`corrFuncType`**Context:**`colvar`**Acceptable Values:**`velocity`,`coordinate`or`coordinate_p2`**Default Value:**`velocity`**Description:**With`coordinate`or`velocity`, the correlation function = is calculated between the variables and , or their velocities. is the scalar product when calculated between scalar or vector values, whereas for quaternions it is the cosine between the two corresponding rotation axes. With`coordinate_p2`, the second order Legendre polynomial, , is used instead of the cosine.Normalize the time correlation function?`corrFuncNormalize`**Context:**`colvar`**Acceptable Values:**boolean**Default Value:**`on`**Description:**If enabled, the value of the correlation function at = 0 is normalized to 1; otherwise, it equals to .Length of the time correlation function`corrFuncLength`**Context:**`colvar`**Acceptable Values:**positive integer**Default Value:**`1000`**Description:**Length (in number of points) of the time correlation function.Stride of the time correlation function`corrFuncStride`**Context:**`colvar`**Acceptable Values:**positive integer**Default Value:**`1`**Description:**Number of steps between two values of the time correlation function.Offset of the time correlation function`corrFuncOffset`**Context:**`colvar`**Acceptable Values:**positive integer**Default Value:**`0`**Description:**The starting time (in number of steps) of the time correlation function (default: = 0).**Note:***the value at = 0 is always used for the normalization*.Output file for the time correlation function`corrFuncOutputFile`**Context:**`colvar`**Acceptable Values:**UNIX filename**Default Value:**`name .corrfunc.dat`**Description:**The time correlation function is saved in this file.Calculate the running average and standard deviation`runAve`**Context:**`colvar`**Acceptable Values:**boolean**Default Value:**`off`**Description:**Whether or not the running average and standard deviation should be calculated for this colvar.Length of the running average window`runAveLength`**Context:**`colvar`**Acceptable Values:**positive integer**Default Value:**`1000`**Description:**Length (in number of points) of the running average window.Stride of the running average window values`runAveStride`**Context:**`colvar`**Acceptable Values:**positive integer**Default Value:**`1`**Description:**Number of steps between two values within the running average window.Output file for the running average and standard deviation`runAveOutputFile`**Context:**`colvar`**Acceptable Values:**UNIX filename**Default Value:**`name .runave.dat`**Description:**The running average and standard deviation are saved in this file.