  Quantum mechanical many-electron calculations can predict properties of
atoms, molecules and even complex materials. The employed computational methods
play a quintessential role in many scientifically and technologically relevant
research fields. However, a question of paramount importance is whether
approximations aimed at reducing the computational complexity for solving the
many-electron Schr\"odinger equation, are accurate enough. Here, we investigate
recently reported discrepancies of noncovalent interaction energies for large
molecules predicted by two of the most widely-trusted many-electron theories:
diffusion quantum Monte Carlo and coupled-cluster theory. We are able to
unequivocally pin down the source of the puzzling discrepancies and present
modifications to widely-used coupled-cluster methods needed for more accurate
noncovalent interaction energies of large molecules on the hundred-atom scale.
This enhances the reliability of predictions from quantum mechanical
many-electron theories across a wide range of critical applications, including
drug design, catalysis, and the innovation of new functional materials, such as
those for renewable energy technologies.
