  Atoms under pressure undergo a series of processes and modification of its
electronic structure. Examples are the spontaneous ionization, stabilization of
excited-state configurations that result in a level-crossing with the ground
state of the free atom, and contraction of atomic shells. In this work, we do a
systematic study of the effects of confinement with harmonic potential on the
electronic structure of atoms from H to Ne. Dynamic and static correlation is
taken into account by performing CCSD and CASSCF calculations, respectively.
Because the strength of harmonic confinement cannot be translated into
pressure, we envisioned a "calibration" method to transform confinement into
pressure. We focused on the effect of confinement on: i) changes of electron
distribution and localization within the $K$ and $L$ atomic shells, ii)
confinement-induced ionization pressure, iii) level crossing of electronic
states, and iv) the electron correlation energy. We found that contraction of
valence and core shells are not negligible and that the use of standard
pseudopotentials might be not adequate to study solids under extreme pressures.
The critical pressure at which and atom ionizes follows a periodic trend, and
it ranges from $28$ GPa for Li to $10.8$ TPa for Ne. In Li and a Be, pressure
induces mixing of the ground state configuration with excited states. At high
pressure, the ground state of Li and Be becomes a doublet and a triplet with
configurations $1s^22p$ and $1s^22s2p$ respectively. The potential consequences
of these changes of configuration on the chemistry of Be are discussed.
Finally, the changes in the amount of electron correlation are characterized
and analyzed in terms of the RPA approximation. For atoms with fewer electrons
in the valence shell correlation increases, but for atoms with more electron,
the increasing of kinetic energy dominates over electron correlation.
