  The multi-component quantum theory of atoms in molecules (MC-QTAIM) analysis
is done on methane, ethylene, acetylene and benzene as selected basic
hydrocarbons. This is the first report on applying the MC-QTAIM analysis on
polyatomic species. In order to perform the MC-QTAIM analysis, at first step
the nuclear-electronic orbital method at Hartree-Fock level (NEO-HF) is used as
a non-Born-Oppenheimer (nBO) ab initio computational procedure assuming both
electrons and protons as quantum waves while carbon nuclei as point charges in
these systems. The ab initio calculations proceed substituting all the protons
of each species first with deuterons and then tritons. At the next step, the
derived nBO wavefunctions are used for the "atoms in molecules" (AIM) analysis.
The results of topological analysis and integration of atomic properties
demonstrate that the MC-QTAIM is capable of deciphering the underlying AIM
structure of all the considered species. Also, the results of the analysis for
each isotopic composition are distinct and the fingerprint of the mass
difference of hydrogen isotopes is clearly seen in both topological and AIM
analyses. This isotopic distinction is quite unique in the MC-QTAIM and not
recovered by the orthodox QTAIM that treats nuclei as clamped particles. The
results of the analysis also demonstrate that each quantum nucleus that forms
an atomic basin resides within its own basin. The confinement of quantum nuclei
within a single basin is used to simplify the basic equations of the MC-QTAIM
paving the way for future theoretical studies.
