Journal article

A generalization of the effective-charge concept: Dynamical multipoles in molecular solids and liquids

The charge "belonging" to a given atom in a condensed system is not a measurable quantity, and can be defined only by means of some arbitrary model and/or theory: the static dipole (or multipole) associated with a given molecule in a molecular solid or liquid is analogously ill-defined. At variance with this, the dynamical (alias infrared, IR) atomic charge is a well defined quantity, much studied in recent years for various materials: it measures the coupling of atomic displacements to macroscopic electric fields, either static or in the IR regime. We present here the generalization of the concept of atomic dynamical charge addressing the dynamical monopole and dipole for a molecule in condensed-either solid or liquid-molecular phase. In the low frequency IR region, dominated by intermolecular modes (translational and librational), such monopole and dipole are responsible for the shape of the spectrum. For an isolated molecule the dynamical monopole and dipole both coincide with the static ones: the changes occurring when embedding a given molecule in a condensed environment carry in compact form outstanding information about the intermolecular interactions. We present first-principle results for the test case of liquid water, where the effects of such interactions are particularly dramatic.


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