Simple introduction to MD

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The smallest constituents of all matter that surrounds us are atoms. An atom consists of a very small (10^-15 m) nucleus, which has a positive electric charge, and a number (1-110) of electrons surrounding the nucleus. In an atom the charge of the nucleus is equal to the charge of the surrounding electrons. If the charges are not equal, scientists talk about ions. Groups of atoms that are tightly bounded to each other by the interactions of their electrons are called molecules.

Despite their very small size (10^-10 m) there are nowadays ways in which individual atoms can be viewed and in some cases even moved. However, this can only be done in very few circumstances. And there is no way in which one can, for instance, look at the movement of atoms within solid matter.

But what one can do nowadays is to use a computer to simulate the movements of a group of atoms or molecules. Methods which do this are due to historical reasons called molecular dynamics simulations. The forces which act between atoms are known to an accuracy of about 10% or better, and this enables phycists and chemists to calculate the movement of atoms with a fairly good precision.

The solving is done by calculating all significant forces that act between atoms when they are at given positions. When the forces are calculated, one can use them to calculate how much and in what directions the atoms will move during a very short time (of the order of 10^-15 seconds). Once the atoms have been given their new positions, the process is continued by calculating forces in the new positions.

The simulations usually need very much computer capacity. And even with the most effective computers available today it is not possible to calculate the movement of more than perhaps a million atoms at a time. This is very little, considering a millimeter long chunk of a human hair contains perhaps 10^18 atoms, a million million times more. Also, it is not possible to simulate processes that last more than minute fractions of a second.

In spite of these limitations, molecular dynamics simulations can be used to examine and describe numerous problems in physics and chemistry. They have been used to study chemical reactions, manufacture processes of computer chips, the melting of solids, the spreading of spray paint drops on a surface and the hardness properties of metals, just to name a few examples.

Unfortunately, I'm not aware of any works of popular science that would discuss molecular dynamics processes, so I can't really recommend any further reading. But take a look at the color pictures and animations in the other links around here.

In them, each dot or ball depicts a single atom. The old atom positions are not erased between time steps to give a better picture of the atom paths.