  We present a generalized version of the ITIM algorithm for the identification
of interfacial molecules, which is able to treat arbitrarily shaped interfaces.
The algorithm exploits the similarities between the concept of probe sphere
used in ITIM and the circumsphere criterion used in the alpha-shapes approach,
and can be regarded either as a reference-frame independent version of the
former, or as an extended version of the latter that includes the atomic
excluded volume. The new algorithm is applied to compute the intrinsic
orientational order parameters of water around a DPC and a cholic acid micelle
in aqueous environment, and to the identification of solvent-reachable sites in
four model structures for soot. The additional algorithm introduced for the
calculation of intrinsic density profiles in arbitrary geometries proved to be
extremely useful also for planar interfaces, as it allows to solve the paradox
of smeared intrinsic profiles far from the interface
