Title: Ab initio exploration of short-pitch skyrmion materials: Role of orbital frustration

Abstract: In recent years, the skyrmion lattice phase with a short lattice constant has attracted attention due to its high skyrmion density, making it a promising option for achieving high-density storage memory and for observing novel phenomena like the quantized topological Hall effect. Unlike conventional non-centrosymmetric systems where the Dzyaloshinsky-Moriya interaction plays a crucial role, the short pitch skyrmion phase requires a quadratic magnetic interaction $J(q)$ with a peak at finite-$Q$, and weak easy-axis magnetic anisotropy is also critical. Thus, conducting first-principles evaluations is essential for understanding the formation mechanism as well as for promoting the discovery of new skyrmion materials. In this {\it Perspective}, we focus on recent developments of the first-principles evaluations of these properties and apply them to the prototype systems Gd$T_2X_2$ and Eu$T_2X_2$, where $T$ denotes a transition metal and $X$ represents Si or Ge. In particular, based on the spin density functional theory with the Hubbard correction combined with the Liechtenstein method in the Wannier tight-binding model formalism, we first show that the Hubbard $U$ and Hund's coupling is essential to stabilize a skyrmion lattice state by enhancing the easy-axis anisotropy. We then discuss mechanisms of finite-$Q$ instability and show that competition among Gd-5$d$ orbitals determines whether ferromagnetism or a finite-$Q$ structure is favored in Gd$T_2$Si$_2$ with $T=$ Fe and Ru. Our systematic calculations reveal that GdRu$_2$$X_2$, GdOs$_2$$X_2$, and GdRe$_2X_2$ are promising, while GdAg$_2X_2$, GdAu$_2X_2$, and EuAg$_2X_2$ are possible candidates as the skyrmion host materials. Analysis based on a spin spiral calculation for the candidate materials is also presented.