An ab initio downfolding method is formulated to construct low-dimensional models for correlated electrons. In addition to band downfolding by a constrained random phase approximation formulated for 3D models, screening away from the target layer (chain) is further involved. Eliminating the off-target degrees of freedom, namely, dimensional downfolding, yields ab initio low-dimensional models. The method is applied to derive a 2D model for the layered superconductor LaFeAsO, where interlayer screening crucially makes the effective interaction short-ranged and reduces the onsite Coulomb interactions by 10-20% compared with the 3D model for the five iron-3d orbitals.
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