  Understanding DC electrical conductivity is crucial for the study of
materials. Macroscopic DC conductivity can be calculated from first principles
using the Kubo-Greenwood equation. The procedure involves finding the
thermodynamic limit of the current response to an electric field that is slowly
switched on, and then taking the limit of the switching rate to zero. We
introduce a nonlinear extrapolation procedure executed in systems with periodic
boundary conditions, which predicts conductivity close to the thermodynamic
limit even for very small systems. The scheme also overcomes a large part of
the usual ambiguities of the DC conductivity definition for finite systems. We
numerically compare our method to the Landauer equation-based approach and find
both techniques to be consistent with each other.
