CHARGE-TRANSFER MOLECULAR-DYNAMICS

A scheme is described for the explicit treatment of charge-transfer processes in molecular-dynamics simulations of condensed phases. Technical details of the simulations are discussed, and it is shown that it is possible to implement the method without serious cost in computing time relative to standard molecular-dynamics calculations for charged systems. The scheme used ensures that the charge-transfer forces are dynamically conservative, and no numerical instabilities have been observed in the integration of the equations of motion. The method is applied to pure amorphous silica and to silica to which small numbers of dilithium oxide impurities have been added. The distributions of atomic charges have their expected form in each case. In particular, addition of the cation impurities leads to the spontaneous generation of an almost equal number of oxygen defects, and it is shown that it is possible to distinguish unambiguously between bridging and dangling oxygens on electrostatic rather than geometric grounds. Calculation of the radial distribution function for dangling oxygens with respect to lithium ions yields a curve with a very sharp main peak and an integrated coordination number of approximately one.