Start with a cube whose vertices coincide with the sphere centers. Then subtract
the spheres from it:

difference {

  box { < -d/2, -d/2, -d/2 >, < d/2, d/2, d/2 > }

  sphere { <d/2, d/2, d/2> ,  d/1.4}
  sphere { <-d/2, -d/2, -d/2> ,  d/1.4}
  sphere { <-d/2, d/2, d/2> ,  d/1.4}
  sphere { <d/2, -d/2, -d/2> ,  d/1.4}
  sphere { <d/2, -d/2, d/2> ,  d/1.4}
  sphere { <-d/2, d/2, -d/2> ,  d/1.4}
  sphere { <d/2, d/2, -d/2> ,  d/1.4}
  sphere { <-d/2, -d/2, d/2> ,  d/1.4}

  pigment{ color rgb<1,0,0> }

}

HTH
Steven

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