C(n) = \langle \cos\theta_{i, i+n} \rangle =
           \langle \mathbf{a_i} \cdot \mathbf{a_{i+n}} \rangle
           
where :math:`\mathbf{a}_i \text{ and } \mathbf{a}_{i+n}` are unit vectors
along the bonds. (Because for two vectors a and b, 
:math:`\mathbf{a} \cdot \mathbf{b} = |\mathbf{a}| |\mathbf{b}| \cos{\alpha}`)
– see 
`Wikipedia <https://en.wikipedia.org/wiki/Dot_product#Geometric_definition>`_.