<P> When quantum mechanics was originally formulated, it was applied to models whose correspondence limit was non-relativistic classical mechanics . For instance, the well - known model of the quantum harmonic oscillator uses an explicitly non-relativistic expression for the kinetic energy of the oscillator, and is thus a quantum version of the classical harmonic oscillator . </P> <P> Early attempts to merge quantum mechanics with special relativity involved the replacement of the Schrödinger equation with a covariant equation such as the Klein--Gordon equation or the Dirac equation . While these theories were successful in explaining many experimental results, they had certain unsatisfactory qualities stemming from their neglect of the relativistic creation and annihilation of particles . A fully relativistic quantum theory required the development of quantum field theory, which applies quantization to a field (rather than a fixed set of particles). The first complete quantum field theory, quantum electrodynamics, provides a fully quantum description of the electromagnetic interaction . The full apparatus of quantum field theory is often unnecessary for describing electrodynamic systems . A simpler approach, one that has been employed since the inception of quantum mechanics, is to treat charged particles as quantum mechanical objects being acted on by a classical electromagnetic field . For example, the elementary quantum model of the hydrogen atom describes the electric field of the hydrogen atom using a classical − e 2 / (4 π ε 0 r) (\ displaystyle \ scriptstyle - e ^ (2) / (4 \ pi \ \ epsilon _ (_ (0)) \ r)) Coulomb potential . This "semi-classical" approach fails if quantum fluctuations in the electromagnetic field play an important role, such as in the emission of photons by charged particles . </P> <P> Quantum field theories for the strong nuclear force and the weak nuclear force have also been developed . The quantum field theory of the strong nuclear force is called quantum chromodynamics, and describes the interactions of subnuclear particles such as quarks and gluons . The weak nuclear force and the electromagnetic force were unified, in their quantized forms, into a single quantum field theory (known as electroweak theory), by the physicists Abdus Salam, Sheldon Glashow and Steven Weinberg . These three men shared the Nobel Prize in Physics in 1979 for this work . </P> <P> It has proven difficult to construct quantum models of gravity, the remaining fundamental force . Semi-classical approximations are workable, and have led to predictions such as Hawking radiation . However, the formulation of a complete theory of quantum gravity is hindered by apparent incompatibilities between general relativity (the most accurate theory of gravity currently known) and some of the fundamental assumptions of quantum theory . The resolution of these incompatibilities is an area of active research, and theories such as string theory are among the possible candidates for a future theory of quantum gravity . </P>

What is the basis for the quantom mechanical model of the atom