Trouble with CELL_OPT with unit cells of different sizes

[Du Zhang]

Hi Everyone,
 
I ran into some confusing results when I attempted to perform the following 
geometry optimizations on Si crystal with Si2 and Si8 unit cells. 
 
The input file for Si2:
 

&GLOBAL

PROJECT Si_bulk2

RUN_TYPE CELL_OPT

PRINT_LEVEL MEDIUM

&END GLOBAL

&FORCE_EVAL

METHOD Quickstep

STRESS_TENSOR ANALYTICAL

&SUBSYS

&KIND Si

ELEMENT Si

BASIS_SET DZVP-GTH-PADE

POTENTIAL GTH-PADE-q4

&END KIND

&CELL

A 3.83753393 0.00000000 0.00000000

B 1.91876697 3.32340187 0.00000000

C 1.91876697 1.10780062 3.13333333

SYMMETRY RHOMBOHEDRAL

PERIODIC XYZ

&END CELL

&COORD

Si 1.91876697 1.10780062 0.78333333

Si 0.00000000 -0.00000000 -0.00000000

&END COORD

&END SUBSYS

&DFT

BASIS_SET_FILE_NAME BASIS_SET

POTENTIAL_FILE_NAME GTH_POTENTIALS

&QS

EPS_DEFAULT 1.0E-10

&END QS

&MGRID

NGRIDS 4

CUTOFF 300

REL_CUTOFF 60

&END MGRID

&XC

&XC_FUNCTIONAL PADE

&END XC_FUNCTIONAL

&END XC

&SCF

SCF_GUESS ATOMIC

EPS_SCF 1.0E-7

MAX_SCF 300

ADDED_MOS 10

&DIAGONALIZATION ON

ALGORITHM STANDARD

&END DIAGONALIZATION

&MIXING T

METHOD BROYDEN_MIXING

ALPHA 0.4

NBROYDEN 8

&END MIXING

&SMEAR ON

METHOD FERMI_DIRAC

ELECTRONIC_TEMPERATURE [K] 300

&END SMEAR

&END SCF

&END DFT

&PRINT

&FORCES ON

&END FORCES

&END PRINT

&END FORCE_EVAL

&MOTION

&CELL_OPT

KEEP_ANGLES

KEEP_SYMMETRY

&END CELL_OPT

&GEO_OPT

TYPE MINIMIZATION

MAX_DR 1.0E-03

MAX_FORCE 1.0E-03

RMS_DR 1.0E-03 

RMS_FORCE 1.0E-03

MAX_ITER 200

OPTIMIZER BFGS

!&BFGS

! MAX_STEEP_STEPS 0

! RESTART_LIMIT 9.0E-01

!&END BFGS

&END GEO_OPT

&CONSTRAINT

&FIXED_ATOMS

COMPONENTS_TO_FIX XYZ

LIST 2

&END FIXED_ATOMS

&END CONSTRAINT

&END MOTION 

Its initial cell structure is given by:

CELL| Volume [angstrom^3]: 39.961

CELL| Vector a [angstrom]: 3.838 0.000 0.000 |a| = 3.838

CELL| Vector b [angstrom]: 1.919 3.323 0.000 |b| = 3.838

CELL| Vector c [angstrom]: 1.919 1.108 3.133 |c| = 3.838

CELL| Angle (b,c), alpha [degree]: 60.000

CELL| Angle (a,c), beta [degree]: 60.000

CELL| Angle (a,b), gamma [degree]: 60.000

CELL| Requested initial symmetry: RHOMBOHEDRAL

CELL| Numerically orthorhombic: NO

 

And its final cell structure is

CELL| Volume [angstrom^3]: 137.274

CELL| Vector a [angstrom]: 5.790 0.000 0.000 |a| = 5.790

CELL| Vector b [angstrom]: 2.895 5.015 0.000 |b| = 5.790

CELL| Vector c [angstrom]: 2.895 1.672 4.728 |c| = 5.790

CELL| Angle (b,c), alpha [degree]: 60.000

CELL| Angle (a,c), beta [degree]: 60.000

CELL| Angle (a,b), gamma [degree]: 60.000

CELL| Requested initial symmetry: RHOMBOHEDRAL

CELL| Numerically orthorhombic: NO

 

And the final geometry is

i = 45, E = -7.6936438860

Si 4.1049115417 -0.4237817114 4.7278581917

Si 0.0000000000 0.0000000000 0.0000000000

The Si-Si length is clearly too long.

 

 

 

 

For the Si8 case, things appear quite normal.

Input:

&GLOBAL

 PROJECT Si_bulk8

 RUN_TYPE CELL_OPT

 PRINT_LEVEL MEDIUM

&END GLOBAL

&FORCE_EVAL

 METHOD Quickstep

 STRESS_TENSOR ANALYTICAL

 &SUBSYS

 &KIND Si

 ELEMENT Si

 BASIS_SET DZVP-GTH-PADE

 POTENTIAL GTH-PADE-q4

 &END KIND

 &CELL

 A 5.430697500 0.000000000 0.000000000

 B 0.000000000 5.430697500 0.000000000

 C 0.000000000 0.000000000 5.430697500

 SYMMETRY CUBIC

 PERIODIC XYZ

 &END CELL

 &COORD

 Si 0.000000000 0.000000000 0.000000000

 Si 0.000000000 2.715348700 2.715348700

 Si 2.715348700 2.715348700 0.000000000

 Si 2.715348700 0.000000000 2.715348700

 Si 4.073023100 1.357674400 4.073023100

 Si 1.357674400 1.357674400 1.357674400

 Si 1.357674400 4.073023100 4.073023100

 Si 4.073023100 4.073023100 1.357674400

 &END COORD

 &END SUBSYS

 &DFT

 BASIS_SET_FILE_NAME BASIS_SET

 POTENTIAL_FILE_NAME GTH_POTENTIALS

 &QS

 EPS_DEFAULT 1.0E-10

 &END QS

 &MGRID

 NGRIDS 4

 CUTOFF 300

 REL_CUTOFF 60

 &END MGRID

 &XC

 &XC_FUNCTIONAL PADE

 &END XC_FUNCTIONAL

 &END XC

 &SCF

 SCF_GUESS ATOMIC

 EPS_SCF 1.0E-7

 MAX_SCF 300

 ADDED_MOS 10

 &DIAGONALIZATION ON

 ALGORITHM STANDARD

 &END DIAGONALIZATION

 &MIXING T

 METHOD BROYDEN_MIXING

 ALPHA 0.4

 NBROYDEN 8

 &END MIXING

 &SMEAR ON

 METHOD FERMI_DIRAC

 ELECTRONIC_TEMPERATURE [K] 300

 &END SMEAR

 &END SCF

 &END DFT

 &PRINT

 &FORCES ON

 &END FORCES

 &END PRINT

&END FORCE_EVAL

&MOTION

 &CELL_OPT

 KEEP_ANGLES

 KEEP_SYMMETRY

 &END CELL_OPT

 &GEO_OPT

 TYPE MINIMIZATION

 MAX_DR 1.0E-03

 MAX_FORCE 1.0E-03

 RMS_DR 1.0E-03 

 RMS_FORCE 1.0E-03

 MAX_ITER 200

 OPTIMIZER BFGS

 !&BFGS

 ! MAX_STEEP_STEPS 0

 ! RESTART_LIMIT 9.0E-01

 !&END BFGS

 &END GEO_OPT

 &CONSTRAINT

 &FIXED_ATOMS

 COMPONENTS_TO_FIX XYZ

 LIST 1

 &END FIXED_ATOMS

 &END CONSTRAINT

&END MOTION 

 Final cell structure:

CELL| Volume [angstrom^3]: 173.162

CELL| Vector a [angstrom]: 5.574 0.000 0.000 |a| = 5.574

CELL| Vector b [angstrom]: 0.000 5.574 0.000 |b| = 5.574

CELL| Vector c [angstrom]: 0.000 0.000 5.574 |c| = 5.574

CELL| Angle (b,c), alpha [degree]: 90.000

CELL| Angle (a,c), beta [degree]: 90.000

CELL| Angle (a,b), gamma [degree]: 90.000

CELL| Requested initial symmetry: CUBIC

CELL| Numerically orthorhombic: YES
 
Final geometry

i = 4, E = -31.3087094725

Si 0.0000000000 0.0000000000 0.0000000000

Si -0.0000000224 2.7868973667 2.7868973671

Si 2.7868973677 2.7868973683 -0.0000000226

Si 2.7868973673 -0.0000000224 2.7868973679

Si 4.1803460577 1.3934486673 4.1803460580

Si 1.3934486913 1.3934486907 1.3934486902

Si 1.3934486666 4.1803460575 4.1803460574

Si 4.1803460579 4.1803460580 1.3934486678

 

Could anyone help me with this confusion? Thanks a lot!

 

 

 

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