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:

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

\(k_i\) (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

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 input for this action is the scalar output from one or more other actions. 
=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 input for this action is the scalar output from one or more other actions. 
=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
the input for this action is the scalar output from one or more other actions. 
=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

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...