Libraries of Extremely Localized Molecular Orbitals. 1. Model Molecules Approximation and Molecular Orbitals Transferability

Despite more and
more remarkable computational ab initio results are
nowadays continuously obtained for large macromolecular
systems, the development of new linear-scaling techniques is still
an open and stimulating field of research in theoretical chemistry.
In this family of methods, an important role is occupied by those
strategies based on the observation that molecules are generally constituted
by recurrent functional units with well-defined intrinsic features.
In this context, we propose to exploit the notion of extremely localized
molecular orbitals (ELMOs) that, due to their strict localization
on small molecular fragments (e.g., atoms, bonds, or functional groups),
are in principle transferable from one molecule to another. Accordingly,
the construction of orbital libraries to almost instantaneously build
up approximate wave functions and electron densities of very large
systems becomes conceivable. In this work, the ELMOs transferability
is further investigated in detail and, furthermore, suitable rules
to construct model molecules for the computation of ELMOs to be stored
in future databanks are also defined. The obtained results confirm
the reliable transferability of the ELMOs and show that electron densities
obtained from the transfer of extremely localized molecular orbitals
are very close to the corresponding Hartree–Fock ones. These
observations prompt us to construct new ELMOs databases that could
represent an alternative/complement to the already popular pseudoatoms
databanks both for determining electron densities and for refining
crystallographic structures of very large molecules.