Integrated Multiscale Multilevel Approach to Open Shell Molecular Systems

We present a novel multiscale approach to study the electronic
structure of open shell molecular systems embedded in an external
environment. The method is based on the coupling of multilevel Hartree–Fock
(MLHF) and Density Functional Theory (MLDFT), suitably extended to
the unrestricted formalism, to Molecular Mechanics (MM) force fields
(FF). Within the ML region, the system is divided into active and
inactive parts, thus describing the most relevant interactions (electrostatic,
polarization, and Pauli repulsion) at the quantum level. The surrounding
MM part, which is formulated in terms of nonpolarizable or polarizable
FFs, permits a physically consistent treatment of long-range electrostatics
and polarization effects. The approach is extended to the calculation
of hyperfine coupling constants and applied to selected nitroxyl radicals
in an aqueous solution.