In order to describe a system we need four fundamental features:

• System description - What are the fundamental units or "particles", and how many are there?

• Starting condition - Where are the particles and what are their velocities?

• Interaction - What is the mathematical form for the forces acting between the particles?

• Dynamical equation - What is the mathematical form for evolving the system in time?

The choice of "particles" puts limitations on what we are ultimately able to describe. If we choose atomic nuclei and electrons as our building blocks, we can describe atoms and molecules, but not the internal structure of the atomic nucleus. If we choose atoms as the building blocks, we can describe molecular structures, but not the details of the electron distribution. If we choose amino acids as the building blocks,we may be able to describe the overall structure of a protein, but not the details of atomic movements.

Electrons — |

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