<div dir="ltr">Dear All, There have been some discussions about electric fields recently and a possible bug has been spotted on the calculation of the forces in the qs_efield_utils.F routine concerning fields applied for isolated molecules. Here I post another issue that may be related to those already discussed concerning periodic fields. While running a DFT-MD of a liquid water-solid interface under the presence of a periodic electric field of 0.5 V/A along +x, I noticed that the waters align their dipoles in the opposite direction to the field (that is -x), whereas they should align their dipoles in the same verse of the field. Also the energy (i.e. the conserved quantity in the NVT ensemble using the CSVR thermostat) shows a large drift, of about 0.15 eV/(ps*atom), while the same simulation without the field gives 7*10^(-5) eV/(ps*atom). Here I report some tests on the geometry optimizations of a water molecule with and without the electric field (using PERIODIC_EFIELD) and I found that in the presence of the field the water molecule in optimized configuration has its dipole pointing towards -x instead of towards +x. Not surprisingly when running a simulation with the field towards -x the molecule the reorients with the dipole pointing towards +x... water in no field: ENERGY= -17.222255518668135 a.u. water in periodic field using the section: &PERIODIC_EFIELD INTENSITY 0.0097234 #in au 1 au=51.42207 V/A POLARISATION 1.0 0.0 0.0 &END ENERGY=-17.229857100831254 a.u. I attach the input, coordinates during optimization and the output files for the cases above. I checked in the qs_efield_berry.F file in case the forces on the ions may have a wrong sign as suggested in the post <a href="https://groups.google.com/forum/#%21searchin/cp2k/electric$20field%7Csort:date/cp2k/kH_a6k21FLQ/IZAphZhKtkMJI"> https://groups.google.com/forum/#!searchin/cp2k/electric$20field|sort:date/cp2k/kH_a6k21FLQ/IZAphZhKtkMJ</a> concerning the routines for the EFIELD section, but I could not identify a similar term in the periodic field case (the line where the forces are computed looks quite different in that file). To sum up I have a few questions concerning the periodic field using the berry phase. Why the water dipoles are oriented in the opposite direction of the field? Why is the energy drift so big for the liquid water-solid interface MD compared to the one where there is no field? Is it possible that there is a bug in the forces computed in the case of the periodic field? I hope someone can shed some light on this. Thanks a lot, Gabriele </div>