Calculate a polynomial combination of a set of other variables.

The functional form of this function is

\[ C=\sum_{i=1}^{N_{arg}} c_i (x_i-a_i)^{p_i} \]

The coefficients c, the parameters a and the powers p are provided as vectors.

Notice that COMBINE is not able to predict which will be periodic domain of the computed value automatically. The user is thus forced to specify it explicitly. Use PERIODIC=NO if the resulting variable is not periodic, and PERIODIC=A,B where A and B are the two boundaries if the resulting variable is periodic.

Examples

The following input tells plumed to print the distance between atoms 3 and 5 its square (as computed from the x,y,z components) and the distance again as computed from the square root of the square.

Click on the labels of the actions for more information on what each action computes

The following input tells plumed to add a restraint on the cube of a dihedral angle. Notice that since the angle has a periodic domain -pi,pi its cube has a domain -pi**3,pi**3.

Click on the labels of the actions for more information on what each action computes

=1,3,5,7 The TORSION action with label t calculates a single scalar value

Glossary of keywords and components

Description of components

Quantity

Description

.#!value

a linear compbination

Compulsory keywords

PERIODIC

if the output of your function is periodic then you should specify the periodicity of the function. If the output is not periodic you must state this using PERIODIC=NO

COEFFICIENTS

( default=1.0 ) the coefficients of the arguments in your function

PARAMETERS

( default=0.0 ) the parameters of the arguments in your function

POWERS

( default=1.0 ) the powers to which you are raising each of the arguments in your function

Options

NORMALIZE

( default=off ) normalize all the coefficients so that in total they are equal to one

ARG

the input to this function. You can use multiple instances of this keyword i.e. ARG1, ARG2, ARG3...