Figure 3.

Molecular structures of a series of amino acids (a) and dicarboxylic acids separated by different number of methylene groups (b), and same length dicarboxylic acids but different QM regions (c). The fragments bearing functional groups are treated by Hartree–Fock theory and the methylene groups in parentheses are modeled as molecular mechanics models. The carbon atoms labeled by ${\mathrm{C}}_b$ indicate the position where the boundary atoms are located, treated by the projected hybrid orbital method.