DENPOL: a new program to determine electron densities of polypeptides using extremely localized molecular orbitals

A new method to compute high-quality electron densities of polypeptides is proposed. The method is based on the transferability properties of extremely localized molecular orbitals, which can be used to describe with great accuracy the different functional groups of a molecule. It is therefore possible to generate a database of such orbitals, each of them associated with specific amino acids or with the peptide bond. A new program, DENPOL, has been written in order to build up the electron density of a generic polypeptide using this database. Due to both the large number of orbitals required to describe a polypeptide and the non-orthogonal nature of these orbitals, a Divide & Conquer strategy has been used to assemble the final electron density. The application of this approach is particularly efficient thanks to the extreme localization of the orbitals. The comparison with the corresponding electron densities generated by the Hartree–Fock method, shows the accuracy of the proposed approach and indicates that the electron densities generated by DENPOL are very close to those generated by an ab initio approach