Electronic polarization in the ultrasoft pseudopotential formalism
  An expression is given for the electronic polarization of an insulating crystal within the ultrasoft pseudopotential scheme. The pseudopotential charge-augmentation terms modify the usual Berry-phase expression, and also give rise to a second term that takes the form of a conventional expectation value. 
David Vanderbilt (Department of Physics and Astronomy, Rutgers   University) and R.D. King-Smith (Molecular Simulations Inc....