Electrostatic Function

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Electrostatic Function

The electrostatic function is given by

(4.16)

$\begin{displaymath} f_{ELEC}(R) = \left\{ \begin{array}{llll} Q_{i}Q_{j}\frac{C}... ... 1/R option}\\ 0 &\mbox{for repel option} \end{array}\right. \end{displaymath}$

The electrostatic function is computed using the atomic charges provided by the CHARge specification in the atom statement (see Section 3.1.1). $\varepsilon_{0}$ is the dielectric constant, which can be defined by the EPS statement; $r_{on}$ and $r_{off}$ are defined by the statements CTONNB and CTOFNB respectively (see Section 3.2.1). Because of the need to limit the number of pair interactions and to avoid discontinuities in the forces (to conserve energy during dynamics), several schemes for truncating the electrostatic potential are used. The option introduces an approximate solvent screening term in the dielectric constant by setting the constant equal to $\varepsilon_{0}R$ . The dielectric option was originally developed because the execution of square roots was expensive on certain obsolete computers. There is no physical justification for the 1/R dielectric, but it is still in widespread use, in particular for simulations in vacuum. The shifted option modifies the radial function so that the energy and forces go to zero at the cutoff distance.

Xplor-NIH 2025-03-21