Reciprocal lattice: The three vectors defining lattice translations. Any point on lattice
 linear combination of lattice vectors a_{1},
a_{2}, a_{3}. Reciprocal vectors are defined as vectors
which give the exponential as 1, i.e. multiples of 2 pi.
For this, they must be orthogonal and normalized as indicated.
A vector product is orthogonal to both of the multiplied
vectors.
