LOWER_WALLS

This is part of the bias module

Defines a wall for the value of one or more collective variables, which limits the region of the phase space accessible during the simulation.

The restraining potential starts acting on the system when the value of the CV is greater (in the case of UPPER_WALLS) or lower (in the case of LOWER_WALLS) than a certain limit \(a_i\) (AT) minus an offset \(o_i\) (OFFSET). The expression for the bias due to the wall is given by:

for UPPER_WALLS: \( \sum_i {k_i}((x_i-a_i+o_i)/s_i)^e_i \)

for LOWER_WALLS: \( \sum_i {k_i}|(x_i-a_i-o_i)/s_i|^e_i \)

\(k_i\) (KAPPA) is an energy constant in internal unit of the code, \(s_i\) (EPS) a rescaling factor and \(e_i\) (EXP) the exponent determining the power law. By default: EXP = 2, EPS = 1.0, OFFSET = 0.

Examples

The following input tells plumed to add both a lower and an upper walls on the distance between atoms 3 and 5 and the distance between atoms 2 and 4. The lower and upper limits are defined at different values. The strength of the walls is the same for the four cases. It also tells plumed to print the energy of the walls.

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
uwall: UPPER_WALLS 
ARG
the arguments on which the bias is acting. 
=d1,d2 
AT
compulsory keyword 
the positions of the wall. 
=1.0,1.5 
KAPPA
compulsory keyword 
the force constant for the wall. 
=150.0,150.0 
EXP
compulsory keyword ( default=2.0 )
the powers for the walls. 
=2,2 
EPS
compulsory keyword ( default=1.0 )
the values for s_i in the expression for a wall 
=1,1 
OFFSET
compulsory keyword ( default=0.0 )
the offset for the start of the wall. 
=0,0 The UPPER_WALLS action with label uwall calculates the following quantities:

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


lwall: LOWER_WALLS 
ARG
the arguments on which the bias is acting. 
=d1,d2 
AT
compulsory keyword 
the positions of the wall. 
=0.0,1.0 
KAPPA
compulsory keyword 
the force constant for the wall. 
=150.0,150.0 
EXP
compulsory keyword ( default=2.0 )
the powers for the walls. 
=2,2 
EPS
compulsory keyword ( default=1.0 )
the values for s_i in the expression for a wall 
=1,1 
OFFSET
compulsory keyword ( default=0.0 )
the offset for the start of the wall. 
=0,0 The LOWER_WALLS action with label lwall calculates the following quantities:

 Quantity   
 Description  
lwall.bias
the instantaneous value of the bias potential
lwall.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 
=uwall.bias,lwall.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 
=uwall.bias,lwall.bias The PRINT action with label 
 (See also DISTANCE and PRINT).
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



 AT  
the positions of the wall. The a_i in the expression for a wall.   

 KAPPA  
the force constant for the wall. The k_i in the expression for a wall.   

 OFFSET  
( default=0.0 ) the offset for the start of the wall. The o_i in the expression for a wall.   

 EXP  
( default=2.0 ) the powers for the walls. The e_i in the expression for a wall.   

 EPS  
( default=1.0 ) the values for s_i in the expression for a wall   

Options



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