Point charges and atomic multipole moments of Si and O in amorphous SiO2 for the estimation of the electrostatic field and potential

In order to estimate atomic multipole moments (AMMs) and charges in the model of amorphous SiO₂ a hybrid B3LYP functional with 30% of the Hartree-Fock Hamiltonian in the exchange part with the 88-31G*(Si)/8-411G*(O) basis set and the CRYSTAL06 package are used. A 192-atomic unit cell of amorphous SiO₂ is chosen as a model, the calculations with which agree well with the experimental static factor of neutron scattering. The second optimized model of amorphous SiO₂ (a-SiO₂) with a smaller number of defects is prepared with the use of the VASP package and full optimization of the initial a-SiO₂ model. For both models the atomic charges and AMMs are calculated (up to the fourth order included) and their approximation is performed. The approximation quality is compared for these models and with a model for crystalline systems whose AMMs were previously calculated. The conclusions are drawn about the applicability of charge and AMM estimates within the approaches such as the embedded cluster.