Function hexagonal_vectors 

pub fn hexagonal_vectors(a_m: f64, c_m: f64) -> [[f64; 3]; 3]

The hexagonal primitive vectors a1 = (a/2, -sqrt(3) a/2, 0), a2 = (a/2, sqrt(3) a/2, 0), a3 = (0, 0, c) (Mehl et al. 2017, pp. 423, 425, 427).