Electronic structure of first and second row atoms under harmonic confinement

Abstract

Atoms under pressure undergo a number of modifications of their electronic structure. Good examples are the spontaneous ionization, stabilization of excited‐state configurations, and contraction of atomic‐shells. In this work, we study the effects of confinement with harmonic potentials on the electronic structure of atoms from H to Ne. Dynamic and static correlation is taken into account with coupled cluster with single and double excitations and CASSCF calculations. 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: (a) changes of electron distribution and localization within the K and L shells, (b) confinement‐induced ionization pressure, (c) level crossing of electronic states, and (d) 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 atoms ionize follows a periodic trend, and it ranges from 28 GPa for Li to 10.8 TPa for Ne. In Li and Be, pressure induces mixing of the ground state configuration with excited states. At high pressure, the ground states of Li and Be become a doublet and a triplet with configurations 1s22p and 1s22s2p, respectively, which could change the chemistry of Be. Finally, it is observed that atoms with fewer electrons correlation increases, but for atoms with more electrons, the increasing of kinetic energy dominates over electron correlation.

Non‐Bonded Radii of the Atoms Under Compression

“…The ways in which compression of individual atoms or molecules have been computed in physics and chemistry are many, [82–101] and will not be reviewed here in detail. In this work, we rely on the eXtreme Pressure Polarizable Continuum Model (XP‐PCM) [102,103] …”

The change in the nature of bonding in the Li2 dimer under confinement

“…The electronic properties of atoms and molecules under confinement is the result of an interplay between attractive and repulsive terms in the Hamiltonian and Pauli's principle that increases the kinetic energy as the available volume is reduced. For a regime of strong confinement, the increasing of the kinetic energy is usually the dominating factor [14, 15].…”

“…For instance, the filling of shells can diverge from the traditional aufbau rule [16]. Also, atoms can be “ionized” by confinement [1, 2, 14]; that is, the cation becomes more stable than the neutral. That property has been used by Hoffmann et al [17] to construct a theory of high pressure electrides.…”

Electronic structure calculations of compressed Li atom using composite technique under Ritz variational framework

“…In another work, Sarsa et al [29] adopted a “Parameterized optimized effective potential” (POEP) technique for confined atoms which was a variational approach with a trial function similar to the HF method with an additional constraint that the orbitals were assumed to be the eigenfunctions of a single‐particle Hamiltonian with a local potential. Robles‐Navarro et al [30] and Rodriguez‐Bautista et al [31] studied the case of confined Li by soft and hard potentials in the framework of CASSCF and Hatree–Fock (HF) method, respectively. All these works [22–24, 28–31] show the effort of researchers to go around the ab initio correlated calculations of many‐electron atoms and take resort to approximate methods and techniques in order to avoid the mathematical and computational barriers.…”

“…Robles‐Navarro et al [30] and Rodriguez‐Bautista et al [31] studied the case of confined Li by soft and hard potentials in the framework of CASSCF and Hatree–Fock (HF) method, respectively. All these works [22–24, 28–31] show the effort of researchers to go around the ab initio correlated calculations of many‐electron atoms and take resort to approximate methods and techniques in order to avoid the mathematical and computational barriers.…”