  Five physics mechanisms of interaction leading to the binding of the ${\rm
H}_3^+$ molecular ion are identified. They are realized in a form of
variational trial functions and their respective total energies are calculated.
Each of them provides subsequently the most accurate approximation for the
Born-Oppenheimer (BO) ground state energy among (two-three-seven)-parametric
trial functions being correspondingly, H$_2$-molecule plus proton (two
variational parameters), H$_2^+$-ion plus H-atom (three variational parameters)
and generalized Guillemin-Zener (seven variational parameters). These trial
functions are chosen following a criterion of physical adequacy. They include
the electronic correlation in the exponential form $\sim\exp{(\gamma r_{12})}$,
where $\gamma$ is a variational parameter. Superpositions of two different
mechanisms of binding are investigated and a particular one, which is a
generalized Guillemin-Zener plus H$_2$-molecule plus proton (ten variational
parameters), provides the total energy at the equilibrium of $E=-1.3432$\ a.u.
The superposition of three mechanisms: generalized Guillemin-Zener plus (H$_2$
-molecule plus proton) plus (H$_2^+$ -ion plus H) (fourteen parameters) leads
to the total energy which deviates from the best known BO energy to $\sim
0.0004$\ a.u., {\it it reproduces two-three significant digits in exact, non-BO
total energy}. In general, our variational energy agrees in two-three-four
significant digits with the most accurate results available at present as well
as major expectation values.
