 This is part of the colvar module 
Calculate the radius of gyration, or other properties related to it.
With this version of the command you can use any property you so choose to define the weights that are used when computing the average. If you use the mass or if all the atoms are ascribed weights of one PLUMED defaults to GYRATION_FAST
 Examples
 Glossary of keywords and components
 Description of components
Quantity  Description 
.#!value  the radius that was computed from the weights 
 The atoms involved can be specified using
ATOMS  the group of atoms that you are calculating the Gyration Tensor for. For more information on how to specify lists of atoms see Groups and Virtual Atoms 
 Compulsory keywords
TYPE  ( default=RADIUS ) The type of calculation relative to the Gyration Tensor you want to perform 
 Options
NOPBC  ( default=off ) ignore the periodic boundary conditions when calculating distances 
PHASES  ( default=off ) use trigonometric phases when computing position of center of mass 
MASS  ( default=off ) calculate the center of mass 
MASS_WEIGHTED  ( default=off ) set the masses of all the atoms equal to one 
UNORMALIZED  ( default=off ) do not divide by the sum of the weights

WEIGHTS  what weights should be used when calculating the center. If this keyword is not present the geometric center is computed. If WEIGHTS=@Masses is used the center of mass is computed. If WEIGHTS=@charges the center of charge is computed. If the label of an action is provided PLUMED assumes that that action calculates a list of symmetry functions that can be used as weights. Lastly, an explicit list of numbers to use as weights can be provided 