What is entropy?

Today, I’ll start a new series of posts, where I talk about some of the concepts important for how FAH works, and also highlighted in our recent youtube posts.  Entropy is a good place to start.  It’s something which is often can lead to very counter intuitive results if you don’t think about it.  Basically, entropy is a measure of how common a given state is.  The importance of entropy is given by temperature: at high temperatures, entropy is very important, and at low temperatures energy is important. 

Check out the movie, where Dan Ensign explains more.

Consider water, which can be a gas (water vapor), liquid (liquid
water), or solid (ice).  As a gas, there are lots of ways of arranging
water molecules (high entropy), as a solid there are very few ways (low
entropy), and liquid is in the middle.  Thus, as one goes from high to
low temperature, one goes through gas, then liquid, then solid phases.

Similar issues are relevant for proteins, which can have different
phases equivalent to sold, liquid, and gas (folded, molten globule, and
unfolded), and it’s entropy which pushes the protein from one phase to