  'Oriented external electric fields (OEEFs)' have been shown to have great
potential in being able to provide unprecedented control of chemical reactions,
catalysis and selectivity with applications ranging from H2 storage to
molecular machines. We report a theoretical study of the atomic origins of
molecular changes due to OEEFs; understanding the characteristics of
OEEF-induced couplings between atomic and molecular properties is an important
step toward comprehensive understanding of the effects of strong external
fields on molecular structure, stability, and reactivity. We focus on the
atomic and molecular (bond) properties of a set of homo- (H2, N2, O2, F2, and
Cl2) and hetero-diatomic (HF, HCl, CO, and NO) molecules under intense external
electric fields in the context of quantum theory of atoms in molecules (QTAIM).
It is shown that atomic properties (atomic charges and energies, and
localization index) correlate linearly with the field strengths, but molecular
properties (bond length, electron density at bond critical point, bond length,
and electron delocalization index) exhibit non-linear responses to the imposed
fields. In particular, the changes in the electron density distribution alter
the shapes and locations of the zero-flux surfaces, atomic volumes, atomic
electron population, and localization/delocalization indices. At the molecular
level, the topography and topology of the molecular electrostatic potential
undergo dramatic changes. The external fields also perturb the
covalent-polar-ionic characteristic of the studied chemical bonds, hallmarking
the impact of electric fields on the stability and reactivity of chemical
compounds. The findings are well-rationalized within the framework of the
quantum theory of atoms in molecules and form a coherent conceptual
understanding of these effects in prototypical molecules such as diatomics.
