  We introduce the notion of electronic enthalpy for first-principles
structural and dynamical calculations of finite systems under pressure. An
external pressure field is allowed to act directly on the electronic structure
of the system studied via the ground-state minimization of the functional
$E+PV_{q}$, where $V_{q}$ is the quantum volume enclosed by a charge
isosurface. The Hellmann-Feynman theorem applies, and assures that the ionic
equations of motion follow an isoenthalpic dynamics. No pressurizing medium is
explicitly required, while coatings of environmental ions or ligands can be
introduced if chemically relevant. We apply this novel approach to the study of
group-IV nanoparticles during a shock wave, highlighting the significant
differences inthe plastic or elastic response of the diamond cage under load,
and their potential use as novel nanostructured impact-absorbing materials.
