calibrate {<aria-calibrate-statement>} end

<aria-calibrate-statement> :==

  automatic
  --{get references from structures}
  dreference <real>   
  --{explicit reference distance}
  errset
  --{determine mean error and set bounds}
  exponent
  --{calibration exponent}
  mode (isotropic|anisotropic)
  --{isotropic, anisotropic, volume dep.}
  reset 
  --{initialize table}
  vector <atom-selection> <atom-selection>
  --{calibration from ... to}
  vreference <real> 
  --{reference volume}
  --{the general format is:
         vector <atom-selection> <atom-selection> auto
     or
         vector <atom-selection> <atom-selection> dref 2.5 vref ...}
