You might have noticed that from time to time constants are specified using strings rather than numbers. An example is the following

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

#SETTINGS MOLFILE=regtest/basic/rt65/AA.pdb
MOLINFO STRUCTUREcompulsory keyword 
a file in pdb format containing a reference structure. 
=AA.pdb MOLTYPEcompulsory keyword ( default=protein )
what kind of molecule is contained in the pdb file - usually not needed since protein/RNA/DNA
are compatible 
=rna The MOLINFO action with label 
e1: TORSION ATOMSthe four atoms involved in the torsional angle 
=@epsilon-1 The TORSION action with label e1 calculates a single scalar value
t: METAD ARGthe input for this action is the scalar output from one or more other actions. 
=e1 SIGMAcompulsory keyword 
the widths of the Gaussian hills 
=0.15 PACEcompulsory keyword 
the frequency for hill addition 
=10 HEIGHTthe heights of the Gaussian hills. 
=2 GRID_MINthe lower bounds for the grid 
=-pi GRID_MAXthe upper bounds for the grid 
=pi GRID_BINthe number of bins for the grid 
=200 The METAD action with label t calculates a single scalar value

Notice that the boundaries for GRID_MIN and GRID_MAX are -pi and pi. Until PLUMED 2.3, we used a very dummy parses that could recognize only pi as a special string, plus strings such as 0.5pi and -pi. However, as of version 2.4, we use the Lepton library in order to parse every constant that we read. This means that you can also employ more complicated expressions such as 1+2 or exp(10):

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

#SETTINGS MOLFILE=regtest/basic/rt65/AA.pdb
MOLINFO STRUCTUREcompulsory keyword 
a file in pdb format containing a reference structure. 
=AA.pdb MOLTYPEcompulsory keyword ( default=protein )
what kind of molecule is contained in the pdb file - usually not needed since protein/RNA/DNA
are compatible 
=rna The MOLINFO action with label 
e1: TORSION ATOMSthe four atoms involved in the torsional angle 
=@epsilon-1 The TORSION action with label e1 calculates a single scalar value
RESTRAINT ARGthe input for this action is the scalar output from one or more other actions. 
=e1 ATcompulsory keyword 
the position of the restraint 
=1+0.5 The RESTRAINT action with label

Notice that this applies to any quantity read by plumed as a real number, but does not apply yet to integer numbers (e.g.: the PACE argument of METAD).