approximation methods needed obtain meaningful predictions ref name perdew ref thomas fermi theory developed used estimate system energy electronic density treating local electron density variational parameter later douglas hartree vladimir fock john slater developed called hartree fock wavefunction improvement thomas fermi model hartree fock method accounted exchange statistics single particle electron wavefunctions general 's difficult solve hartree fock equation free electron gas case solved exactly ref name ashcroftmermin ref finally walter kohn pierre hohenberg
Natural science Physics Condensed matter physics