Density functional calculations on the relative stability of c(4 x 2) and p(2 x 2) reconstructed Si(001) surfaces exposed to external electric fields and charge injection are presented. Electric fields parallel to the [0 0 1] direction or electrons inserted into surface states are found to favor the p(2 x 2) over the c(4 x 2) reconstruction. This explains recent experimental findings for Si and Ge(001). (c) 2005 Elsevier B.V. All rights reserved.