<P> Intermolecular forces (IMFs) are the forces which mediate interaction between molecules, including forces of attraction or repulsion which act between molecules and other types of neighboring particles, e.g., atoms or ions . Inter-molecular forces are weak relative to intramolecular forces--the forces which hold a molecule together . For example, the covalent bond, involving sharing electron pairs between atoms, is much stronger than the forces present between neighboring molecules . Both sets of forces are essential parts of force fields frequently used in molecular mechanics . </P> <P> The investigation of inter-molecular forces starts from macroscopic observations which indicate the existence and action of forces at a molecular level . These observations include non-ideal - gas thermodynamic behavior reflected by virial coefficients, vapor pressure, viscosity, superficial tension, and absorption data . </P>

What do you mean by intermolecular force of attraction