On the accuracy of the 3-body fragment molecular orbital method (FMO) applied to density functional theory

The 3-body fragment molecular orbital method (FMO) was combined with density functional theory (DFT). The accuracy of the new method was tested on water clusters and alanine n-mers (α-helices and β-strands), using B3LYP and the 6-31G* basis set (and 6-31++G** for water clusters). At the best level (3-body, two molecules (residues) per fragment), absolute errors in energy were at most 0.0088 a.u., absolute errors in RMS energy gradients were at most 0.00020 a.u./bohr, and relative errors in RMS dipole moments were at most 6.1 (0.56)%, where the value in parentheses is restricted to the 6-31G* basis set.