I know that the constant $a$ represents the attractive forces between the molecules while the constant $b$ represents the size/repulsion between them. Does that mean that $a$ and $b$ are "inversely proportional" or are they independent of each other? If we have two gases and we can compare their sizes and attractive natures, can we compare $a$ and $b$ for the two gases? If $a$ is greater for gas 1 will $b$ always be lesser for gas 1 as compared to gas 2?