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-2

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

contract shortcuts

d1: DISTANCE 
ATOMS
the pair of atom that we are calculating the distance between. 
=3,5 The DISTANCE action with label d1 calculates a single scalar value
d2: DISTANCE 
ATOMS
the pair of atom that we are calculating the distance between. 
=2,4 The DISTANCE action with label d2 calculates a single scalar value
restraint: RESTRAINT 
ARG
the arguments on which the bias is acting. 
=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 The RESTRAINT action with label restraint calculates the following quantities:

 Quantity   
 Description  
restraint.bias
the instantaneous value of the bias potential
restraint.force2
the instantaneous value of the squared force due to this bias potential


PRINT 
ARG
the input for this action is the scalar output from one or more other actions. 
=restraint.bias The PRINT action with label 

d1: DISTANCE 
ATOMS
the pair of atom that we are calculating the distance between. 
=3,5 The DISTANCE action with label d1 calculates a single scalar value
d2: DISTANCE 
ATOMS
the pair of atom that we are calculating the distance between. 
=2,4 The DISTANCE action with label d2 calculates a single scalar value
restraint: RESTRAINT_SCALAR 
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 The RESTRAINT_SCALAR action with label restraint calculates the following quantities:

 Quantity   
 Description  
restraint.bias
the instantaneous value of the bias potential
restraint.force2
the instantaneous value of the squared force due to this bias potential


PRINT 
ARG
the input for this action is the scalar output from one or more other actions. 
=restraint.bias The PRINT action with label 

Glossary of keywords and components

Description of components

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

ARG	the arguments on which the bias is acting. You can use multiple instances of this keyword i.e. ARG1, ARG2, ARG3...