Slow converge of cutoff

[sungen... at gmail.com]

Hello everyone, 

 I am trying to calculate a system of  water interacting with graphene 

 But when I am trying to find the converged cutoff, and rel_cutoff.  I 
encounter a problem on converging the cutoff

 This is my template input file, only the DFT part.
 18   &DFT
 19     BASIS_SET_FILE_NAME BASIS_MOLOPT
 20     POTENTIAL_FILE_NAME GTH_POTENTIALS
 21     &MGRID
 22       &RS_GRID
 23        DISTRIBUTION_TYPE REPLICATED
 24       &END RS_GRID
 25       COMMENSURATE
 26       NGRIDS 4
 27       CUTOFF LT_cutoff
 28       REL_CUTOFF LT_rel_cutoff
 29     &END MGRID
 30     &QS
 31      EXTRAPOLATION PS
 32      EXTRAPOLATION_ORDER 3
 33      !EPS_DEFAULT 1.0E-12
 34      EPS_DEFAULT  1.0E-10
 35      MAP_CONSISTENT .True.
 36     &END QS
 37     &SCF
 38         EPS_SCF 5.0E-7
 39         MAX_SCF 1
 40       &DIAGONALIZATION
 41          ALGORITHM STANDARD
 42       &END DIAGONALIZATION
 43     &END SCF
 44     &XC
 45       &XC_FUNCTIONAL BLYP
 46       &END XC_FUNCTIONAL
 47        &XC_GRID
 48          XC_DERIV SPLINE2_smooth
 49        &END XC_GRID
 50     &END XC
51   &END DFT


This is my result of converge testing,  

  1 # Grid cutoff vs total energy
  2 # Date: Thu Apr  9 09:51:35 CST 2015
  3 # REL_CUTOFF = 80
  4 # Cutoff (Ry) | Total Energy (Ha) | NG on grid 1 | NG on grid 2 | NG on 
grid 3 | NG on grid 4
  5      50.00  -1077.7986090189  407916    1152       0       0
  6     100.00  -1076.5761227522  363137   45931       0       0
  7     150.00  -1076.3152273239  336168   72468     432       0
  8     200.00  -1076.6215215033  302212  105704    1152       0
  9     250.00  -1076.6215215034  286908  117283    4877       0
 10     300.00  -1076.6918197451  247337  142757   18974       0
 11     350.00  -1076.7443959649  237912  132152   39004       0
 12     400.00  -1076.8075935224  227524  135613   45931       0
 13     450.00  -1076.8216130185  221745  134954   51937     432
 14     500.00  -1076.8656781424  206885  141016   60735     432
 15     550.00  -1076.8656781424  204215  141154   63267     432
 16     600.00  -1076.8931179327  201144  135024   72468     432
 17     650.00  -1076.9259357756  198534  116396   93706     432
 18     700.00  -1076.9259357756  193403  116678   97835    1152
 19     750.00  -1076.9554843710  173654  133643  100619    1152
 20     800.00  -1076.9677290602  165100  137112  105704    1152

It's obviously that, the energy do not converge even with as high as 800.0 
Ry, 

So, what' is the reason ?

Thanks in advance !
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