4TH MOMENT APPROXIMATION TO TIGHT-BINDING - APPLICATION TO BCC TRANSITION-METALS

Using Carlsson's approach of the low-order moments approximation to ti ght binding, we develop improved potentials for Mo and W, also a poten tial for V. This model was fit to twelve bulk properties, namely cohes ive energy, lattice constant, elastic constants, vacancy properties, b cc - fee and bcc - A15 structural energy differences and four zone edg e phonons. Also, for Mo and W, the functions were required to yield a reconstruction of the (100) surface, although the details of the surfa ce were not fit. The potentials were tested by calculating surface pro perties. Most importantly, both the W(100) (root 2 x root 2)R45 degree s and Mo(100) c(7 root 2 x root 2)R45 degrees surface reconstructions are observed, which is a good indication of the physical correctness o f the angular terms, although for Mo(100) the lowest energy occurred o n c(5 root 2 X root 2)R45 degrees structure.