The Electronic Structure-Based Explicit Polarization X-Pol Potential

The Electronic Structure-Based Explicit Polarization (X-Pol) Potential 

Za (/) is the nucleus charge of atom a and 2M is the number of electrons in molecule /, V) ( P/) is the electrostatic potential of molecular J at either the electronic (t = i) or nuclear (t = a) positions of molecule I. The electrostatic potential due to molecule J is defined as follows  

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  

For proteins, the fragment is defined by the peptide unit convention as recommended by the International Union of Pure and Applied Chemistry, although the 

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