I think the correct answer would be intermolecular interactions. The van der waals equation corrects the ideal gas law for the finite volume and intermolecular interactions of gas molecules. The ideal gas law assumes that gas molecules are point masses that has perfectly elastic collisions and that intermolecular interactions are does not exist. But real gases do not behave as such. They highly deviate from these assumptions. The development of the the van der waals equation paved the way for considering the effects of those being neglected in the ideal gas equation. It accounted for the interactions of the molecules and the volume of the gas molecules.