Subject: what applies force from the mean position of a vibrating molecule?

when any substance is heated up it vibrates(as total energy e=e1+e2+e3+e4 where 
e3 
and e4 are kinetic and potential energies of vibration.) and executes a SHM 
which 
implies that there is a source of force at the mean position of vibration but as 
far as i know there isn't anything there which can pull the molecule which is 
moving away from the mean position back towards the mean position.                          

Re: what applies force from the mean position of a vibrating molecule?