Associated with each lattice simplex Δ is a poset encoding the additive structure of lattice points in the fundamental parallelepiped for Δ. When this poset is an antichain, we say Δ is antichain. For each partition λ of n, we define a lattice simplex Δ_λ having one unimodular facet, and we investigate their associated posets. We give a number-theoretic characterization of the relations in these posets, as well as a simplified characterization in the case where each part of λ is relatively prime to n − 1. We use these characterizations to experimentally study Δ_λ for all partitions of n with n ≤ 73. Further, we experimentally study the prevalence of the antichain property among simplices with a restricted type of Hermite normal form, suggesting that the antichain property is common among simplices with this restriction. Finally, we explain how this work relates to Poincaré series for the semigroup algebra associated with Δ and we prove that this series is rational when Δ is antichain.