RESTRAINT
This is part of the bias module

Adds harmonic and/or linear restraints on one or more variables.

Either or both of SLOPE and KAPPA must be present to specify the linear and harmonic force constants respectively. The resulting potential is given by:

\[ \sum_i \frac{k_i}{2} (x_i-a_i)^2 + m_i*(x_i-a_i) \]

.

The number of components for any vector of force constants must be equal to the number of arguments to the action.

Additional material and examples can be also found in the tutorial Lugano tutorial: Using restraints

Examples

The following input tells plumed to restrain the distance between atoms 3 and 5 and the distance between atoms 2 and 4, at different equilibrium values, and to print the energy of the restraint

Click on the labels of the actions for more information on what each action computes
tested on master
d1: DISTANCE 
ATOMS
the pair of atom that we are calculating the distance between.
=3,5 d2: DISTANCE
ATOMS
the pair of atom that we are calculating the distance between.
=2,4 restraint: RESTRAINT
ARG
the input for this action is the scalar output from one or more other actions.
=d1,d2
AT
compulsory keyword the position of the restraint
=1.0,1.5
KAPPA
compulsory keyword ( default=0.0 ) specifies that the restraint is harmonic and what the values of the force constants on each of the variables are
=150.0,150.0 PRINT
ARG
the input for this action is the scalar output from one or more other actions.
=restraint.bias
Glossary of keywords and components
Description of components

By default this Action calculates the following quantities. These quantities can be referenced elsewhere in the input by using this Action's label followed by a dot and the name of the quantity required from the list below.

Quantity Description
bias the instantaneous value of the bias potential
force2 the instantaneous value of the squared force due to this bias potential
Compulsory keywords
SLOPE ( default=0.0 ) specifies that the restraint is linear and what the values of the force constants on each of the variables are
KAPPA ( default=0.0 ) specifies that the restraint is harmonic and what the values of the force constants on each of the variables are
AT the position of the restraint
Options
NUMERICAL_DERIVATIVES

( default=off ) calculate the derivatives for these quantities numerically

ARG the input for this action is the scalar output from one or more other actions. The particular scalars that you will use are referenced using the label of the action. If the label appears on its own then it is assumed that the Action calculates a single scalar value. The value of this scalar is thus used as the input to this new action. If * or *.* appears the scalars calculated by all the proceeding actions in the input file are taken. Some actions have multi-component outputs and each component of the output has a specific label. For example a DISTANCE action labelled dist may have three components x, y and z. To take just the x component you should use dist.x, if you wish to take all three components then use dist.*.More information on the referencing of Actions can be found in the section of the manual on the PLUMED Getting Started. Scalar values can also be referenced using POSIX regular expressions as detailed in the section on Regular Expressions. To use this feature you you must compile PLUMED with the appropriate flag. You can use multiple instances of this keyword i.e. ARG1, ARG2, ARG3...