Libraries of Extremely Localized Molecular Orbitals. 2. Comparison with the Pseudoatoms Transferability

Due to both technical
and methodological difficulties, determining
and analyzing charge densities of very large molecular systems represents
a serious challenge that, in the crystallographers community, has
been mainly tackled by observing that the so-called pseudoatoms of
the electron density multipole expansions are reliably transferable
from molecule to molecule. This has led to the construction of pseudoatoms
databanks that have allowed successful refinements of crystallographic
structures of macromolecules, while taking into account their corresponding
reconstructed electron distributions. A recent alternative/complement
to the previous approach is represented by techniques based on extremely
localized molecular orbitals (ELMOs) that, due to their strict localization
on small molecular fragments (e.g., atoms, bonds, and functional groups),
are also in principle exportable from system to system. The ELMOs
transferability has been already tested in detail, and, in this work,
it has been compared to the one of the pseudoatoms. To accomplish
this task, electron distributions obtained both through the transfer
of pseudoatoms and through the transfer of extremely localized molecular
orbitals have been analyzed, especially taking into account topological
properties and similarity indexes. The obtained results indicate that
all the considered reconstruction methods give completely reasonable
and similar charge densities, and, consequently, the new ELMOs libraries
will probably represent new useful tools not only for refining crystal
structures but also for computing approximate electronic properties
of very large molecules.