The surface tension of water droplets consisting of a few tens of molecules has been determined by two novel techniques within a molecular dynamics (MD) framework. Both involve the guided separation of a molecular cluster into fragments; either individual molecules in a process called disassembly, or into equal size subclusters, which we term mitosis. The mechanical work performed during guided separation under isothermal conditions may be related to the free energy change associated with the process, through use of the Jarzynski relation. The free energy change can then be interpreted in terms of the surface tension, or more properly, the excess free energy of cluster formation from the condensed phase. This quantity plays a central role in models of the nucleation of aerosol droplets from supersaturated vapours. Our results at 300 K for clusters up to 128 molecules in size, using the TIP4P/2005 interaction model, yield a surface tension similar to that used in the internally consistent classical theory (ICCT).