about energy calculation error

[chun]

Hi,
    I use cp2k to do all-electron energy calculation on a single
water
molecule with periodic boundry condition and unit cell size
9.4Angstrom X 9.4Angstrom X 9.4Angstrom (see the input file in the
end). When I move the molecule from the center (4.7,4.7,4.7) of the
cell to the corner (2.7,2.7,2.7) with the same structure, there is an
energy difference about 0.5 mili-Hartree between them. Could anyone
tell me what causes the energy difference?

Thanks.

Chun-Min


&FORCE_EVAL
  METHOD Quickstep
  &DFT
    &MGRID
      CUTOFF 158
    &END MGRID
    &QS
      METHOD GAPW
      FULL_GAPW
    &END QS
    &SCF
      SCF_GUESS ATOMIC
    &END SCF
    &XC
      &XC_FUNCTIONAL BLYP
      &END XC_FUNCTIONAL
    &END XC
  &END DFT
  &SUBSYS
    &CELL
      ABC 9.4 9.4 9.4
      UNIT ANGSTROM
    &END CELL
    &COORD
    O   4.700000    4.700000    4.634413
    H   4.700000    3.942864    5.220545
    H   4.700000    5.457136    5.220545
#    O   2.700000    2.700000    2.634413
#    H   2.700000    1.942864    3.220545
#    H   2.700000    3.457136    3.220545
    &END COORD
    &KIND H
      BASIS_SET 6-31G*
      POTENTIAL ALL
    &END KIND
    &KIND O
      BASIS_SET 6-31G*
      POTENTIAL ALL
    &END KIND
  &END SUBSYS
&END FORCE_EVAL
&GLOBAL
  PROJECT H2O
  PRINT_LEVEL MEDIUM
&END GLOBAL