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For a complete description of the physical properties of low-energy QCD, itmight be advantageous to first reformulate QCD in terms of gauge-invariantdynamical variables, before applying any approximation schemes. Using acanonical transformation of the dynamical variables, which Abelianises thenon-Abelian Gauss-law constraints to be implemented, such a reformulation canbe achieved for QCD. The exact implementation of the Gauss laws reduces thecolored spin-1 gluons and spin-1/2 quarks to unconstrained colorless spin-0,spin-1, spin-2 and spin-3 glueball fields and colorless Rarita-Schwinger fieldsrespectively. The obtained physical Hamiltonian can then be rewritten into aform, which separates the rotational from the scalar degrees of freedom, andadmits a systematic strong-coupling expansion in powers of lambda=g^{-2/3},equivalent to an expansion in the number of spatial derivatives. Theleading-order term in this expansion corresponds to non-interactinghybrid-glueballs, whose low-lying masses can be calculated with high accuracyby solving the Schr\"odinger-equation of the Dirac-Yang-Mills quantum mechanicsof spatially constant physical fields (at the moment only for the 2-colorcase). Due to the presence of classical zero-energy valleys of thechromomagnetic potential for two arbitrarily large classical glueball fields(the unconstrained analogs of the well-known constant Abelian fields),practically all glueball excitation energy is expected to go into the increaseof the strengths of these two fields. Higher-order terms in lambda lead tointeractions between the hybrid-glueballs and can be taken into accountsystematically using perturbation theory in lambda.

QCD in terms of gauge-invariant dynamical variables