The average electrostatic potential in the box is set to zero in Ewald summation. Bear in mind that the value of potential is artificial in PBC and has no physical importance. If you have a vacuum phase in the system, the absolute potential scale can be recovered by referencing the potential to the vacuum value provided that there is no internal dipole. Otherwise, try to remove periodicity along some directions using MT method. Jun <div class="gmail_quote">On Thu, Jun 24, 2010 at 2:37 AM, zh <<a href="mailto:vale...@gmail.com">vale...@gmail.com</a>> wrote: <blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;"> Dear all, When "PERIODIC" and "POISSON_SOLVER" are specified with "XYZ" and "PERIODIC", what is the reference point for the zero of energy (i.e., eigenvalue or electrostatic potential)? For a system contains vacuum, is the electrostatic potential in the vacuum region set as zero in CP2K? Thanks for your help. Best regards Zh -- You received this message because you are subscribed to the Google Groups "cp2k" group. To post to this group, send email to <a href="mailto:cp...@googlegroups.com">cp...@googlegroups.com</a>. To unsubscribe from this group, send email to <a href="mailto:cp2k%2Bun...@googlegroups.com">cp2k+uns...@googlegroups.com</a>. For more options, visit this group at <a href="http://groups.google.com/group/cp2k?hl=en" target="_blank">http://groups.google.com/group/cp2k?hl=en</a>. </blockquote></div>