( This ( ( means ( it ( is ( ( the sum ) ( of ( its ( ( ( kinetic and ) potential ) energy ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT This)) (VP (VBZ means) (SBAR (S (NP (PRP it)) (VP (VBZ is) (NP (NP (DT the) (NN sum)) (PP (IN of) (NP (PRP$ its) (ADJP (JJ kinetic) (CC and) (JJ potential)) (NN energy)))))))) (. .)))
This means it is the sum of its kinetic and potential energy.
(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)))) (. .)))