Hey kezman.
Can you explain in detail what Q and v are? (i.e. give a mathematical definition)
That's gibberish as long as we don't know what v_{i} and e_{i} are. I would guess that "e_{i}" are the unit vectors in the x, y, and z directions, but still don't know what "v_{i}" is. And what "parallelepiped" are you talking about.