A RasMol within expression allows atoms to be selected on their proximity to another set of atoms. A within expression takes two parameters separated by a comma and surrounded by parenthesis. The first argument is an integer value called the "cut-off" distance of the within expression and the second argument is any valid atom expression. The cut-off distance is expressed in either integer RasMol units or Angstroms containing a decimal point. An atom is selected if it is within the cut-off distance of any of the atoms defined by the second argument. This allows complex expressions to be constructed containing nested within expressions.
For example, the command select within(3.2,backbone) selects any atom within a 3.2 Angstrom radius of any atom in a protein or nucleic acid backbone. Within expressions are particularly useful for selecting the atoms around an active site.