Model examples

For inorganic materials such as simple ionic crystals, only a few parameters are needed to accurately model the system.

For example, diagram below shows a set of potential parameters for magnesium oxide crystal. It is highly ionic and there is no covalent bond. This means the total energy of the system consists of only the non-bonded components:

../../_images/ff13_1.png

V(total) = V(vdw) + V(coulombic)

with

V(bonded) = 0

The corresponding FF parameters are shown below (in DL_POLY form).

../../_images/ff13_2.png

The magnesium cation is assigned with the charge +2 and the oxide anion with the charge -2 and three vdw parameter sets for Buckingham potential were assigned to account for all three possible pair-wise combinations: Mg…Mg, Mg…O and O…O.

Note

The Mg3 and O10 as shown are just atom labels for the magnesium and oxygen, respectively. Also, there are no vdw interaction for Mg…Mg pair. The pair would repel naturally from each other due to the strong electrostatic charge.

../../_images/Orange_bar2.png

Unlike ionic crystals, FF models for organic molecules contain a lot more parameters since V(bonded) is needed to maintain the structure of the molecule.

Take for example dimethyl sulfoxide, a small organic molecule which is commonly used as a solvent.

../../_images/ff13_3.png

In this case, all bonded interactions would need to take into account.

Below shows part of the potential parameters (shown in DL_POLY’s FIELD file format) for CHARMM FF.

../../_images/ff13_4.png

Note the amount of parameters needed even for a small molecule. While manual entry of parameters can be trivial for simple ionic crystals, large covalent molecules would need some utility programs, such as DL_FIELD, to help setting up the FF models correctly.

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