Frozen density embedding
(FDE) represents an embedding scheme in
which environmental effects are included from first-principles calculations
by considering the surrounding system explicitly by means of its electron
density. In the present paper, we extend the full four-component relativistic
Dirac–Kohn–Sham (DKS) method, as implemented in the
BERTHA code, to include environmental and confinement effects with
the FDE scheme (DKS-in-DFT FDE). The implementation, based on the
auxiliary density fitting techniques, has been enormously facilitated
by BERTHA’s python API (PyBERTHA), which facilitates the interoperability
with other FDE implementations available through the PyADF framework.
The accuracy and numerical stability of this new implementation, also
using different auxiliary fitting basis sets, has been demonstrated
on the simple NH3–H2O system, in comparison
with a reference nonrelativistic implementation. The computational
performance has been evaluated on a series of gold clusters (Au
n
, with n = 2, 4, 8) embedded
into an increasing number of water molecules (5, 10, 20, 40, and 80
water molecules). We found that the procedure scales approximately
linearly both with the size of the frozen surrounding environment
(consistent with the underpinnings of the FDE approach) and with the
size of the active system (in line with the use of density fitting).
Finally, we applied the code to a series of heavy (Rn) and super-heavy
elements (Cn, Fl, Og) embedded in a C60 cage to explore
the confinement effect induced by C60 on their electronic
structure. We compare the results from our simulations, with respect
to more-approximate models employed in the atomic physics literature.
Our results indicate that the specific interactions described by FDE
are able to improve upon the cruder approximations currently employed,
and, thus, they provide a basis from which to generate more-realistic
radial potentials for confined atoms.