Molecular crystals are solids formed when molecules are weakly bound (can der Waals force) together in a lattice pattern or array that repeats throughout the crystal.
Molecules are atoms bound with ionic or covalent bonds that are strong enough to keep the molecule together in solid state, in solution, or in a gaseous state. This force is called intramolecular. The van der Waal or intermolecular forces occur in interactions between the molecules and are not strong enough to break the intramolecular bonds of the molecule. Some examples of molecular crystals are crystal, also known as sugar rock candy and dry ice or solid carbon dioxide.