Electrostatic interactions partial electronic charges

Hqm/mm is often approximated as the electrostatic interactions between the charges (electrons and nuclei) of the QM region and the partial atomic charges on the atoms in the MM region plus the van der Waals interactions between the atoms of the QM region and the... 

Consequently, we introduce the second approximation which is to use an approximate electrostatic potential in Eq.(4-21) to determine inter-fragment electronic interaction energies. Thus, the electronic integrals in Eq. (4-21) are expressed as a multipole expansion on molecule J, whose formalisms are not detailed here. If we only use the monopole term, i.e., partial atomic charges, the interaction Hamiltonian is simply given as follows ... 

The electrostatic interactions between the partial charges in the classical subsystem and the electrons and the nuclei in the QM system are described by this term. We let the term Efff° denote the interactions between the MM partial charges and the QM nuclei and the index s runs over all the sites in the MM system. Typically, the sites in the MM system are located at the MM atoms having the partial charge qs and positioned according to the position vector Rs. 

But we have not told you the whole story about BF3. Boron is in group 3 and thus has only six electrons around it in its trivalent compounds. A molecule of BF3 is planar with an empty p orbital. This is the reverse of a lone pair. An empty orbital on an atom does not repel electron-rich areas of other molecules and so the oxygen atom of acetone is attracted electrostatically to the partial positive charge and one of the lone pairs on oxygen can form a bonding interaction with the empty orbital. We shall develop these ideas in the next section. 

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