  The source HESS J1813-178 was detected in the survey of the inner Galaxy in
TeV gamma-rays, and a SNR G12.8-0.0 was identified in the radio band to be
associated with it. The PWN embedded in the SNR is powered by an energetic
pulsar PSR J1813-1749, which was recently discovered. Whether the TeV
gamma-rays originate from the SNR shell or the PWN is uncertain now. We
investigate theoretically the multiwavelength nonthermal radiation from the
composite SNR G12.8-0.0. The emission from the particles accelerated in the SNR
shell is calculated based on a semianalytical method to the nonlinear diffusive
shock acceleration mechanism. In the model, the magnetic field is
self-generated via resonant streaming instability, and the dynamical reaction
of the field on the shock is taken into account. Based on a model which couples
the dynamical and radiative evolution of a PWN in a non-radiative SNR, the
dynamics and the multi-band emission of the PWN are investigated. The particles
are injected with a spectrum of a relativistic Maxwellian plus a power law
high-energy tail with an index of -2.5. Our results indicate that the radio
emission from the shell can be well reproduced as synchrotron radiation of the
electrons accelerated by the SNR shock; with an ISM number density of 1.4
cm^{-3} for the remnant, the gamma-ray emission from the SNR shell is
insignificant, and the observed X-rays and VHE gamma-rays from the source are
consistent with the emission produced by electrons/positrons injected in the
PWN via synchrotron radiation and IC scattering, respectively; the resulting
gamma-ray flux for the shell is comparable to the detected one only with a
relatively larger density of about 2.8 cm^{-3}. The VHE gamma-rays of HESS
J1813-178 can be naturally explained to mainly originate from the nebula
although the contribution of the SNR shell becomes significant with a denser
ambient medium.
