// Define a parameter for later use
:P Z_HALF_LENGTH 2.5*cm

// Define materials
:ELEM Hydrogen H 1. 1.
:ELEM Oxygen O 8 16.
:MIXT Water 1. 2
      Hydrogen   2./18.
      Oxygen     16./18.

// Define rotation matrix
:ROTM R00 0. 0. 0.  // unit matrix

// Define volumes and place them
:VOLU world BOX 100. 100. 100. G4_AIR

:VOLU "my box" BOX  50. 50. 50. G4_AIR
:PLACE "my box" 1 world R00 0. 0. $Z_HALF_LENGTH

:VOLU Chamber BOX 5.*cm 5.*cm $Z_HALF_LENGTH Water
:PLACE Chamber 1 "my box" R00 0. 0. -$Z_HALF_LENGTH
:PLACE Chamber 2 "my box" R00 0. 0. $Z_HALF_LENGTH