Density function M06 method has been used to optimize the geometries of camptothecin-cytosine at 6-31+G* basis.Finally, thirteen stabilized complexes have been obtained.Theories of atoms in molecules （AIM） and natural bond orbital （NBO） have been utilized to investigate the hydrogen bonds involved in all the complexes.The interaction energies of all the complexes are corrected by basis set superposition error （BSSE）.By the analysis of complexes interaction energy, charge density, second-order interaction energies E（2）, it is indicated that the complex 6 is the most stable structure.
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