  We propose a general method for constructing system-dependent basis functions
for correlated quantum chemical calculations. Our construction combines
features from several traditional approaches: plane waves, localized basis
functions, and wavelets. In a one-dimensional mimic of Coulomb systems, it
requires only 2-3 basis functions per electron to achieve chemical accuracy,
and reproduces the natural orbitals. We illustrate its effectiveness for
molecular energy curves and chains of many atoms. We discuss the promise and
challenges for realistic quantum chemical calculations.
