I don't think we can achieve linear scaling as suggested
 in Baroni&Giannozzi92. In their case the kinetic energy
 was calculated by finite differences, so it's strictly
 localized in R-space.
 Using psinc functions instead, the kinetic energy is still
 localized, but it non-vanishing on further neighbors.
 Mostofi et al. mention that the application of the kinetic
 energy is still a global operation to be performed with
 FFTs (http://www.tcm.phy.cam.ac.uk/onetep/intro/node6.html).
 In other words, if we want to keep the G-space accuracy, 
 we NEED to use the full representation of the kinetic
 energy WITHOUT truncation.
 This means that it is useless to compute the matrix 
 elements of the kinetic energy in the psinc basis, since
 they don't decay so fast.
 For Haydock, we still need a localized object to start
 with, and the psinc choice seems still to be the most
 convenient to switch to/from the G-space.