The SEN Approach to Hydrogen Bond Energies

The basic idea of the shared-electron-number (SEN) method [243] is to estimate the strength of a hydrogen bond by means of only one variable. This variable is the two-center shared-electron number ctha, which is related linearly to the hydrogen bond energy j ha. 

Qualitatively speaking, itha denotes the number of electrons that is shared by the hydrogen atom H and the acceptor atom A. It is calculated in a population analysis [244—246], where electrons are distributed to different atoms according to the contribution of (modified) atomic orbitals (i.e. of a minimum set of basis functions) to the molecular orbitals. The shared-electron number is thus a single-valued descriptor for the electronic density between the atoms H and A. 

Furthermore, it could be shown that these large error margins cover deviations that are systematic with regard to the acceptor atom [247]. This gives rise to different ideas on how to improve the accuracy of the method. A promising Ansatz (extension) is the decomposition into sets with the same acceptor atom. With the introduction of this decomposition of the total set, the standard deviation could be considerably lowered [247]. 

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