The Morse/Long-range potential energy function is of the form V ( r ) = D e ( 1 − − u ( r ) u ( r e ) e − − β β ( r ) y p r e q ( r ) ) 2 where for large r , V ( r ) ≃ ≃ D e − − u ( r ) + u ( r ) 2 4 D e , so u ( r ) is defined according to the theoretically correct long-range behavior expected for the interatomic interaction. D e is the depth of the potential at equilibrium.