( ( ( The ( internal energy ) ) ( of ( a system ) ) ) ( ( is ( ( the ( total ( thermal ( kinetic ( energy ( and ( thermal ( potential energy ) ) ) ) ) ) ) ) ( of ( all ( ( ( its atoms ) and ) molecules ) ) ) ) ) . ) )
(ROOT (S (NP (NP (DT The) (JJ internal) (NN energy)) (PP (IN of) (NP (DT a) (NN system)))) (VP (VBZ is) (NP (NP (DT the) (JJ total) (JJ thermal) (JJ kinetic) (NN energy) (CC and) (JJ thermal) (JJ potential) (NN energy)) (PP (IN of) (NP (PDT all) (NP (PRP$ its) (NNS atoms)) (CC and) (NP (NNS molecules)))))) (. .)))
The internal energy of a system is the  total thermal kinetic energy and thermal potential energy of all its atoms and molecules.
(ROOT (S (NP (NP (DT The) (NN sum)) (PP (IN of) (NP (NP (DT the) (JJ kinetic) (CC and) (JJ potential) (NNS energies)) (PP (IN of) (NP (NP (DT a) (NN system) (POS 's)) (NNS atoms) (CC and) (NNS molecules)))))) (VP (VBZ is) (VP (VBN called) (NP (JJ internal) (NN energy)))) (. .)))