Molecular crystal in a triclinic cell

[Alex]

Hi all,

We're trying to simulate an n-octane crystal (triclinic, coordinates and 
cell vectors from literature) -- just a static cell + geometry 
optimization. Here's the issue: when simulating a single cell (1 1 1), the 
crystal cohesive energy is far above the experiment -- an order of 
magnitude larger in absolute value. As a vacuum ref we're simulating an 
isolated octane molecule. In all cases, a NON_LOCAL vdw energy correction 
is used.

When simulating a total of eight cells (2 2 2), and the simulation is still 
running, so the results are preliminary, things appear to be much more 
reasonable. The input file is below (I can provide the coordinates as well, 
if needed). Can you just by simple inspection tell me if anything is 
obviously wrong?

Thanks a lot,

Alex

**********************************
&GLOBAL
  PROJECT opt
  PRINT_LEVEL MEDIUM 
  RUN_TYPE CELL_OPT
&END GLOBAL
 &MOTION
  &GEO_OPT
    OPTIMIZER BFGS 
  &END
  &CELL_OPT
    EXTERNAL_PRESSURE [bar] 1.0
    OPTIMIZER BFGS
  &END
&END MOTION
 
&FORCE_EVAL
  METHOD QS
  STRESS_TENSOR ANALYTICAL
  &DFT
    BASIS_SET_FILE_NAME BASIS_SET2
    POTENTIAL_FILE_NAME GTH_POTENTIALS2
    &MGRID
      NGRIDS 5
      CUTOFF 500
    &END MGRID
    &QS
      METHOD GPW
    &END QS
    &SCF
      SCF_GUESS RESTART 
      EPS_SCF 5.0E-6
      MAX_SCF 300
      ADDED_MOS  100 
      &MIXING
          METHOD BROYDEN_MIXING
          ALPHA    0.1
          NBROYDEN   8
      &END MIXING
    &END SCF
    &XC
      &XC_FUNCTIONAL PBE
      &END XC_FUNCTIONAL
      &VDW_POTENTIAL NON_LOCAL
      &END VDW_POTENTIAL
    &END XC
  &END DFT
  &SUBSYS
    &CELL
  ABC 4.160 4.750 11.000
ANGLES 94.80  84.50 105.10
        PERIODIC XYZ
SYMMETRY TRICLINIC
        MULTIPLE_UNIT_CELL 2 2 2 
    &END
    &TOPOLOGY
       COORD_FILE_NAME noct_2mol.pdb
       COORD_FILE_FORMAT PDB
       MULTIPLE_UNIT_CELL  2 2 2 
    &END
    &KIND C
      BASIS_SET  DZVP-MOLOPT-SR-GTH
      POTENTIAL  GTH-PBE-q4
    &END KIND
    &KIND H
      BASIS_SET  DZVP-MOLOPT-SR-GTH
      POTENTIAL  GTH-PBE-q1
    &END KIND
&END SUBSYS
&END

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