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 values the harmonic restraint acts upon. 
=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
compulsory keyword 
the labels of the values that you would like to print to the file 
=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
PRINT 
ARG
compulsory keyword 
the labels of the values that you would like to print to the file 
=restraint.bias The PRINT action with label 
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



 ARG  
the values the harmonic restraint acts upon. You can use multiple instances of this keyword i.e. ARG1, ARG2, ARG3...