Basis set superposition error

Okay, let me explain basis set superposition error to you as if you are 5 years old.

So, when scientists want to study molecules (you know, the tiny things that make up everything in the world), they use a special kind of math to understand how they behave and interact with each other. This math is called quantum mechanics.

Now, to do this math, scientists need to use something called a basis set. A basis set is like a set of tools that helps them understand the behavior of the molecules. It's kind of like how you use tools to build a sandcastle.

But here's the thing - sometimes, when scientists use a basis set to study two molecules that are close to each other, the results can be a bit off. This is because the basis set doesn't take into account the fact that the molecules are affecting each other.

This is what we call basis set superposition error.

It's kind of like if you were trying to measure how much sand you had in your bucket, but your brother kept pouring sand from his bucket into yours. The amount of sand you think you have would be different because of your brother's sand.

This error can happen when scientists use a basis set to study how two molecules interact with each other. They might think the molecules are interacting in a certain way, but in reality, the basis set isn't taking into account the fact that the molecules are affecting each other.

So, scientists have to be careful when using basis sets to study molecules that are close to each other. They have to use special techniques to make sure the basis set takes into account the interaction between the molecules. This way, they can get more accurate results.