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By using IGMPlot you can identify, characterize, and quantify molecular interactions over a
broad range: from non-covalent to covalent bonding, through metal
coordination. This tool can
be helpful for interpretation accessible to a wide community of chemists (organic, inorganic
chemistry, including transition metal
complexes and reaction mechanisms).
From a practical perspective, an attractive feature of the IGM approach is to provide an
automatic workflow delivering
properties that provides chemists with a visual and quantitative
understanding of interactions. Although IGMPlot relies on the electron density (ED)
no topological analysis is required like in QTAIM and an IGM analysis can be achieved with
For more detailed information on the
concept please refer to the original papers:
- Lefebvre C., Rubez G., Khartabil H., Boisson JC., Contreras-García J., Hénon E. Phys Chem Chem Phys.
2017, 19, 17928 doi: http://doi.org/10.1039/c7cp02110k
- Lefebvre C., Khartabil H., Boisson JC., Contreras-García J., Piquemal J.-P., Hénon E. Chem. Phys. Chem.
2018, 19, 1 doi: http://doi.org/10.1002/cphc.201701325
- Miguel Ponce-Vargas, Corentin Lefebvre, Jean-Charles Boisson, Eric Hénon J. Chem. Inf. Model. 2020, 60, 1,
268 doi: http://doi.org/10.1021/acs.jcim.9b01016
- Johanna Klein, Hassan Khartabil, Jean-Charles Boisson, Julia Contreras-García, Jean-Philip Piquemal, Eric
Hénon J. . Phys. Chem. A 2020, 124, 9, 1850 doi: http://doi.org/10.1021/acs.jpca.9b09845
IGMPlot is based on the IGM concept and its local descriptor called δg, which can be
calculated from the electron density ρ (ED).
The IGM-δg approach was initially designed to work with promolecular density, to describe
weak interactions. The term promolecular ED refers to an ED model prior to molecule formation.
The promolecular ED is a non-relaxed electron density, the sum of spherically
averaged neutral atomic densities ρi. It
lacks the relaxation introduced in the SCF procedure or in DFT calculations, and then it fails
to describe covalent situations for which a wavefunction description is required.
In the promolecular mode,
only the atomic coordinates have to be supplied and the promolecular ED estimations are
fast compared to Quantum Mechanical calculations.
But in 2018, we proposed the Gradient-Based Partitioning (GBP)
that extends the IGM concept to electron density derived from a wavefunction. Thus,
thanks to the new IGM approach, detailed information can be directly obtained either on
the covalent or on the non-covalent domain, for small and larger molecular systems.
In the absence of interactions, for instance,
in an isolated atom, ρ shows an exponential decay far from the nucleus. In contrast, in
molecular systems, deviations from this exponential decay can be observed. δg captures
these deviations. From a practical perspective, IGMPlot employs a numeric grid and
calculates local δg at each node of the grid. δg is obtained by doing the difference between
the ED gradient of a reference non-interacting system (the so-called Independent Gradient Model)
and the ED gradient of the real system.
More precisely, the δg local descriptor quantifies the contragradience between the two fragment ED sources. The so-called ED contragradience
situation indicates the mutual penetration of electron charge densities, which is expected
when fragments sources are brought closer to each other, and is also sometimes referred to hereafter as
density overlap or density sharing. The IGM
definition grants that positions of space with non-zero values of δg exclusively correspond to interaction situations
and the greater δg, the stronger
the interaction. δg is not dimensionless.
In order to better highlight peaks in δg(ρ) plots, a new qg descriptor has been devised to
color points of this plot. It is the ’quotient’ twin of the 'difference'
δg descriptor. It is dimensionless and it is very sensitive to ED contragradience situation, but it cannot
serve to quantify the interaction since it tends to infinity as ∇ρ approaches 0 close to critical
points, even for weak interactions like hydrogen-bonding. From a practical perspective, this
descriptor qg serves to color IGM and NCI plots and it is also employed in IGMPlot to find
The IGM framework as described above gives rise to a number of analysing and interpretative
tools in IGMPlot:
- Interaction fingerprint: the IGM-δg descriptor carries information on regions
of space where the total electron
density results from an overlap of individual atomic or molecular electron distributions. By
plotting δg = f(ρ) collected on the nodes of a grid encompassing the molecule,
a specific two-dimensional “fingerprints”
emerges with peaks corresponding to chemical interactions that cover both covalent and
- Regions of interaction: Points with non-zero values of δg
exclusively correspond to interaction situations. Mapping
them back to real space highlights 3D regions associated with molecular interactions by
chemists. Thus, constructing iso-surfaces of δg enables the identification and visualization of
regions of interaction
- Quantifying interactions: the IGM reference gives a unique definition of the
interaction region: non-zero values of δg
exclusively correspond to overlap regions between several ED sources. As a consequence,
the IGM approach overcomes difficulties to define regions within integration procedures to
quantify interactions. In IGMPLot, the quantification of the detected interactions has been
implemented by summing local δg quantities over the space representing the interactions,
leading to different scores.
- A new way for probing bond strength : IGMPlot is able to provide a so-called Intrinsic Bond Strength Index (IBSI) stemming from a new δg_pair descriptor. The IBSI is dimensionless and is normalized to 1 for H2.
Our work has
shown that the IBSI is not a bond order (like Mulliken, Wiberg, Mayer, delocalization index
or ELF). Thus, the values of IBSI should not be compared to those of bond orders. Rather,
it is a new complementary index, related to the bond strength (not to the number of electron pair shared between two atoms).
- Pair Density Asymmetry (PDA): IGMPlot provides
users with the PDA index, which captures the asymmetry of the electron density (ED) distribution attached
to an atom pair. This tool provides a relatively simple way to assess inductive electronic
effects in molecules.
- Atomic decomposition : IGMPlot proposes an atomic decomposition scheme in order to quantify
locally the contribution
of each atom At (δg_inter/At) to the interaction between two fragments A and B
- Atomic Degree Of Interaction (DOI): the DOI index captures any manifestation
of electron density sharing around an atom. It reflects the strength of attachment of an atom
to its molecular neighbourhood. It could be used as input in the context of artificial intelligence for
predicting chemical properties, or even for predicting the outcome of reactions that require
information on the strength of the bonds between atoms. In other respects,
index provides an atomic-level interpretation of reaction mechanism, completing the very
nice picture brought by the benchmark URVA tool based on the internal coordinates.
- Critical Point search : A feature of IGMPlot is to perform a critical point (cp) search
(bonc critical points, ring critical points, cage critical points).
Determining such critical points in a
molecule is of high importance in many studies aiming at characterizing bonds by properties
at cp like energy densities (kinetic, potential) or the ED Laplacian, or ellipticity, ...
For more detailed information we refer the interested reader to the original articles.