I performed DFT calculations for a box of 64 water molecules under Periodic Boundary Condition using CP2K. I found that the forces on each atom do not sum up to zero, as shown in the attached figure. But the total force should be zero due to translational symmetry of the system. Any idea of what ...

I'm using DFT+U implemented in VASP to calculate the electronic density of states of anatase TiO2. In order to more accurately reproduce the experimental bandgap of anatase TiO2 (as compared to standard PBE functional), a U value of 4 eV was applied to the Ti d orbital by adding the settings belo...