  An ab initio downfolding method is formulated to construct low-dimensional
models for correlated electrons. In addition to the band downfolding by
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 a
layered superconductor LaFeAsO, where the interlayer screening crucially makes
the effective interaction short ranged and reduces the onsite Coulomb
interactions by 10-20 \% from the 3D model for the 5 iron-3d orbitals.
