Geometry versus Topology: Testing Self-Consistency of the NiceGraph Program

Molecular structure carries a great amount of information: from simple parameters, such as the number and nature of atoms and bonds, to the more elusive notions of conformation and shape.' An ideal model used to describe molecular structure should be self-contained and should identify the structure in question uniquely. Furthermore, it should be as simple as possible and thus irredundant. The quest for such amodel remains one of the foremost goals of chemical information science. The endeavour to describe molecular structure with graphs fits into the above scheme although it is wellknown that additional informa tion is needed in most cases and that there . are structural features not suitable for graph-theoretical description. Graphs are abstract mathematical objects suitable for describing the molecules, namely the connectivity of their constituent atoms. All geometrical information is missing in such a description. However, a part of molecular geometry could be recovered. An example is offered by Huckel Molecular Orbital (HMO) model where the HMO bond orders are able to give some in-