Treatment of Long-Range Forces

An issue with all force fields is lack of atom-atom polarization. The polarization energy is made up of the charge-dipole interaction, the dipole-dipole interaction, and the energy necessary to induce the dipoles in the sys-tem.3o Unfortunately, inclusion of polarization at each molecular dynamics (MD) time step significantly increases the calculation Therefore, most 

FEP calculations reported in the literature were carried out without explicit inclusion of polarization contributions. Despitp missing polarization contributions, free enei ies calculated by means of molecular mechanics force fields generally are in good agreement with experimental results. One reason may be that molecular mechanics force field parameters are derived such that polarization contributions are included implicitly. Another reason may be that polarization errors cancel during calculations of relative free energy differences for two similar systems. 

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The discussion focuses on two broad aspects of electrical phenomena at interfaces in the first we determine the consequences of the presence of electrical charges at an interface with an electrolyte solution, and in the second we explore the nature of the potential occurring at phase boundaries. Even within these areas, frequent reference will be made to various specialized treatises dealing with such subjects rather than attempting to cover the general literature. One important application, namely, to the treatment of long-range forces between surfaces, is developed in the next chapter. 

Perera, L., Essmann, U., Berkowitz, M. Effect of treatment of long-range forces on the dynamics of ions in aqueous solutions. J. Chem. Phys. 102 (1995) 450-456. 

Tlierc are two major sources of error associated with the calculation of free energies fi computer simulations. Errors may arise from inaccuracies in the Hamiltonian, be it potential model chosen or its implementation (the treatment of long-range forces, e j lie second source of error arises from an insufficient sampling of phase space. 

In the case of histamine, inclusion of this reaction field correction was found to change the result by almost 4 kJ/mol. Unfortunately, the highly sensitive nature of the histamine monocation results to protocol and parameters used meant that no definite conclusion could be drawn about the importance of long-range forces in these calculations. It does however appear that they do play a role. The correct treatment of long-range forces is... 

As soon as the concentration of the solute becomes finite, the coulombic forces between the ions begin to play a role and we obtain both the well-known relaxation effect and an electrophoretic effect in the expression for the conductivity. In Section V, we first briefly recall the semi-phenomenological theory of Debye-Onsager-Falkenhagen, and we then show how a combination of the ideas developed in the previous sections, namely the treatment of long-range forces as given in Section III and the Brownian model of Section IV, allows us to study various microscopic... 

W. F. van Gunsteren and H. J. C. Berendsen, in Molecular Dynamics and Protein Structure, J. Hermans, Ed., Polycrystal Book Service, Western Springs, IL, 1985, pp. 18-22. Treatment of Long Range Forces in Molecular Dynamics. 

Boundary Conditions and the Treatment of Long-Range Forces... 

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