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2. I.N. Levine, Quantum Chemistry, 4th edn., Prentice-Hall, Englewood Cliffs, NJ, 1991.
3. R.F.W. Bader, Atoms in Molecules: A Quantum Theory, Clarendon Press, Oxford, 1990.
4. A topological theory of molecular structure
5. A novel definition of a molecule in a crystal