Thermodynamically it is not possible. Internal energy of a system can not change until and unless an external work is done on the system or by the system.
But I came across a very strange process of Barrier Penetration observed in situations involving nuclear fusion. It seems to violate a very important law of thermodynamics - the law of conservation of energy. If proven true, the internal energy of the system can change by itself.
Here's a link to how : http://psi.phys.wits.ac.za/teaching/Connell/phys284/2005/lecture-02/lecture_02/node12.html