This is part of the function module

Calculate a polynomial combination of a set of other variables.

The functional form of this function is

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

The coefficients c and 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.

Compulsory keywords
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 proceding 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 componets 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.
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
POWERS ( default=1.0 ) the powers to which you are raising each of the arguments in your function
NUMERICAL_DERIVATIVES ( default=off ) calculate the derivatives for these quantities numerically

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

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.
COMBINE  LABEL=distance2 ARG=dist.x,dist.y,dist.z POWERS=2,2,2 PERIODIC=NO
PRINT ARG=distance,distance2
(See also PRINT and DISTANCE).

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.

t: TORSION ATOMS=1,3,5,7
c: COMBINE ARG=t POWERS=3 PERIODIC=-31.0062766802998,31.0062766802998