I was just reading a book on low-temperature physics and stumbled upon a graph that shows the energy of liquid Helium-4 in relation to its molar volume. The graph includes both zero-point energy and the potential energy of the liquid, and the latter goes steeply (basically a vertical line) from being positive for the smallest of molar volumes to being negative, and then gradually (non-linearly) increases as molar volume increases.
So I was wondering what exactly is this potential energy and why is it negative? I've found something online that suggest this potential energy represents pressure. Since I just finished my first year of undergrad, I'm not aware of all the forces between atoms, so I can't really explain why this energy would be more negative with smaller molar volumes. Do they repel each other, and repulsion as opposed to attraction is taken to be negative?
Thanks in advance for any help.