The rich potential energy surface of the water undecamer, (H2O)11 was explored with a basin-hoppping algorithm using a TIP4P potential followed by an ab initio RI-MP2 minimization. The search yielded 24, 71, and 125 distinct minima within 0.5, 1.0 and 2.0 kcal mol-1 of the complete basis set RI-MP2 global minimum, respectively. These minima were categorized into fourteen different groups on the basis of their oxygen framework and hydrogen bonding topolology. Determination of the global minimum was found to be challenging for ab initio methods as well as model potentials due to the presence of many nearly isoenergetic isomers. The predicted global minimum not only varied between ab initio methods and model potentials, but it was highly sensitive to the choice of energy extrapolation schemes, higher-order electron correlation corrections, inclusion of zero-point vibrational energy, and nature of the model potential. The presence of a large number of nearly degenerate structures and isomerization between them manifested itself in anomalous broadening of the heat capacity curve of the undecamer in the melting region. The predicted ensemble of low energy isomers are compared against experimental rotational broadband spectra further suggesting the coexistence of many isomers even at very low temperatures.

Exploring the rich potential energy surface of the water undecamer

248th National Meeting (2014)

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