The structural changes occuring in vitreous silica at high temperatures have been investigated through molecular dynamics simulations made with 'soft potentials', which have weaker electrostatic interactions than other potentials. At ambient pressure, the calculated density shows a maximum close to the temperature at which it has been experimentally reported. With increasing pressure this maximum disappears at around 1.0GPa. The simulations also shows a unusual temperature dependence of the bulk modulus of vitreous silica which increases up to around 800 K and then remains approximately constant from 800 to 1900K. We analyse the local structure by employing two new structural indices which serve to identify four types of entities named alpha-,beta-,gamma- and delta-'structons'.
