Ground state

In physics,ground states ofsystemits lowest-energy quantum mechanical states.

If more than one lowest-energy state exists, theysaidbe degenerate. Many systems have degenerate ground states,example,hydrogen atom. It turns out that degeneracy occurs whenevernontrivial unitary operator commutes withHamiltonian ofsystem.

According tothird lawthermodynamics,system at absolute zero temperature existsits ground state; thus, its entropydetermined bydegeneracy ofground state. Many systems, such as crystal lattices, haveunique ground statetherefore have zero entropy at at absolute zero (because ln(1) = 0).

See also:

• quantum number
• vacuum
• virtual particle