Abstract:The convex hull of n + a nely independent vertices of the unit n-cube I n is called a / -simplex.It is nonobtuse if none its dihedral angles is obtuse, and acute if additionally none of them is right. In terms of linear algebra, acute / -simplices in I n can be described by nonsingular / -matrices P of size n × n whose Gramians G = P P have an inverse that is strictly diagonally dominant, with negative o -diagonal entries [6,7]. The rst part of this paper deals with giving a detailed description of how to e ciently compute, by means of a computer program, a representative from each orbit of an acute / -simplex under the action of the hyperoctahedral group Bn [17] of symmetries of I n . A side product of the investigations is a simple code that computes the cycle index of Bn, which can in explicit form only be found in the literature [11] for n ≤ . Using the computed cycle indices for B , . . . , B in combination with Pólya's theory of enumeration shows that acute / -simplices are extremely rare among all / -simplices. In the second part of the paper, we study the / -matrices that represent the acute / -simplices that were generated by our code from a mathematical perspective. One of the patterns observed in the data involves unreduced upper Hessenberg / -matrices of size n × n, block-partitioned according to certain integer compositions of n. These patterns will be fully explained using a so-called One Neighbor Theorem [4]. Additionally, we are able to prove that the volumes of the corresponding acute simplices are in one-to-one correspondence with the part of Kepler's Tree of Fractions [1,24] that enumerates Q ∩ ( , ). Another key ingredient in the proofs is the fact that the Gramians of the unreduced upper Hessenberg matrices involved are strictly ultrametric [14,26] matrices.