<div dir="ltr"><div>Hi Everyone,</div><div> </div><div>I ran into some confusing results when I attempted to perform the following geometry optimizations on Si crystal with Si2 and Si8 unit cells. </div><div> </div><div>The input file for Si2:</div><div> </div><span lang="EN"><p>&GLOBAL</p><div>
</div><p>  PROJECT Si_bulk2</p><div>
</div><p>  RUN_TYPE CELL_OPT</p><div>
</div><p>  PRINT_LEVEL MEDIUM</p><div>
</div><p>&END GLOBAL</p><div>
</div><p>&FORCE_EVAL</p><div>
</div><p>  METHOD Quickstep</p><div>
</div><p>  STRESS_TENSOR ANALYTICAL</p><div>
</div><p>  &SUBSYS</p><div>
</div><p>    &KIND Si</p><div>
</div><p>      ELEMENT      Si</p><div>
</div><p>      BASIS_SET DZVP-GTH-PADE</p><div>
</div><p>      POTENTIAL GTH-PADE-q4</p><div>
</div><p>    &END KIND</p><div>
</div><p>    &CELL</p><div>
</div><p> A                 3.83753393    0.00000000    0.00000000</p><div>
</div><p> B                 1.91876697    3.32340187    0.00000000</p><div>
</div><p> C                 1.91876697    1.10780062    3.13333333</p><div>
</div><p>      SYMMETRY RHOMBOHEDRAL</p><div>
</div><p>      PERIODIC XYZ</p><div>
</div><p>    &END CELL</p><div>
</div><p>    &COORD</p><div>
</div><p> Si                 1.91876697    1.10780062    0.78333333</p><div>
</div><p> Si                 0.00000000   -0.00000000   -0.00000000</p><div>
</div><p>    &END COORD</p><div>
</div><p>  &END SUBSYS</p><div>
</div><p>  &DFT</p><div>
</div><p>    BASIS_SET_FILE_NAME  BASIS_SET</p><div>
</div><p>    POTENTIAL_FILE_NAME  GTH_POTENTIALS</p><div>
</div><p>    &QS</p><div>
</div><p>      EPS_DEFAULT 1.0E-10</p><div>
</div><p>    &END QS</p><div>
</div><p>    &MGRID</p><div>
</div><p>      NGRIDS 4</p><div>
</div><p>      CUTOFF 300</p><div>
</div><p>      REL_CUTOFF 60</p><div>
</div><p>    &END MGRID</p><div>
</div><p>    &XC</p><div>
</div><p>      &XC_FUNCTIONAL PADE</p><div>
</div><p>      &END XC_FUNCTIONAL</p><div>
</div><p>    &END XC</p><div>
</div><p>    &SCF</p><div>
</div><p>      SCF_GUESS ATOMIC</p><div>
</div><p>      EPS_SCF 1.0E-7</p><div>
</div><p>      MAX_SCF 300</p><div>
</div><p>      ADDED_MOS 10</p><div>
</div><p>      &DIAGONALIZATION  ON</p><div>
</div><p>        ALGORITHM STANDARD</p><div>
</div><p>      &END DIAGONALIZATION</p><div>
</div><p>      &MIXING  T</p><div>
</div><p>        METHOD BROYDEN_MIXING</p><div>
</div><p>        ALPHA 0.4</p><div>
</div><p>        NBROYDEN 8</p><div>
</div><p>      &END MIXING</p><div>
</div><p>      &SMEAR ON</p><div>
</div><p>        METHOD FERMI_DIRAC</p><div>
</div><p>        ELECTRONIC_TEMPERATURE [K] 300</p><div>
</div><p>      &END SMEAR</p><div>
</div><p>    &END SCF</p><div>
</div><p>  &END DFT</p><div>
</div><p>  &PRINT</p><div>
</div><p>    &FORCES ON</p><div>
</div><p>    &END FORCES</p><div>
</div><p>  &END PRINT</p><div>
</div><p>&END FORCE_EVAL</p><div>
</div><p>&MOTION</p><div>
</div><p>  &CELL_OPT</p><div>
</div><p>    KEEP_ANGLES</p><div>
</div><p>    KEEP_SYMMETRY</p><div>
</div><p>  &END CELL_OPT</p><div>
</div><p>  &GEO_OPT</p><div>
</div><p>    TYPE MINIMIZATION</p><div>
</div><p>    MAX_DR    1.0E-03</p><div>
</div><p>    MAX_FORCE 1.0E-03</p><div>
</div><p>    RMS_DR    1.0E-03    </p><div>
</div><p>    RMS_FORCE 1.0E-03</p><div>
</div><p>    MAX_ITER 200</p><div>
</div><p>    OPTIMIZER BFGS</p><div>
</div><p>    !&BFGS</p><div>
</div><p>    !  MAX_STEEP_STEPS  0</p><div>
</div><p>    !  RESTART_LIMIT 9.0E-01</p><div>
</div><p>    !&END BFGS</p><div>
</div><p>  &END GEO_OPT</p><div>
</div><p>  &CONSTRAINT</p><div>
</div><p>    &FIXED_ATOMS</p><div>
</div><p>      COMPONENTS_TO_FIX XYZ</p><div>
</div><p>      LIST 2</p><div>
</div><p>    &END FIXED_ATOMS</p><div>
</div><p>  &END CONSTRAINT</p><div>
</div><p>&END MOTION </p><p>Its initial cell structure is given by:</p><span lang="EN"><p> CELL| Volume [angstrom^3]:                                               39.961</p><p> CELL| Vector a [angstrom]:       3.838     0.000     0.000    |a| =       3.838</p><p> CELL| Vector b [angstrom]:       1.919     3.323     0.000    |b| =       3.838</p><p> CELL| Vector c [angstrom]:       1.919     1.108     3.133    |c| =       3.838</p><span lang="EN"><p> CELL| Angle (b,c), alpha [degree]:                                       60.000</p><p> CELL| Angle (a,c), beta  [degree]:                                       60.000</p><p> CELL| Angle (a,b), gamma [degree]:                                       60.000</p><p> CELL| Requested initial symmetry:                                  RHOMBOHEDRAL</p><p> CELL| Numerically orthorhombic:                                              NO</p></span><p> </p><p>And its final cell structure is</p></span><span lang="EN"><p>CELL| Volume [angstrom^3]:                                              137.274</p><p> CELL| Vector a [angstrom]:       5.790     0.000     0.000    |a| =       5.790</p><p> CELL| Vector b [angstrom]:       2.895     5.015     0.000    |b| =       5.790</p><p> CELL| Vector c [angstrom]:       2.895     1.672     4.728    |c| =       5.790</p><p> CELL| Angle (b,c), alpha [degree]:                                       60.000</p><p> CELL| Angle (a,c), beta  [degree]:                                       60.000</p><p> CELL| Angle (a,b), gamma [degree]:                                       60.000</p><p> CELL| Requested initial symmetry:                                  RHOMBOHEDRAL</p><p> CELL| Numerically orthorhombic:                                              NO</p><p> </p><p>And the final geometry is</p><span lang="EN"><p> i =       45, E =        -7.6936438860</p><p> Si         4.1049115417       -0.4237817114        4.7278581917</p><p> Si         0.0000000000        0.0000000000        0.0000000000</p><p>The Si-Si length is clearly too long.</p><p> </p><p> </p><p> </p><p> </p><p>For the Si8 case, things appear quite normal.</p><p>Input:</p><span lang="EN"><p>&GLOBAL</p><p>
</p><p>  PROJECT Si_bulk8</p><p>
</p><p>  RUN_TYPE CELL_OPT</p><p>
</p><p>  PRINT_LEVEL MEDIUM</p><p>
</p><p>&END GLOBAL</p><p>
</p><p>&FORCE_EVAL</p><p>
</p><p>  METHOD Quickstep</p><p>
</p><p>  STRESS_TENSOR ANALYTICAL</p><p>
</p><p>  &SUBSYS</p><p>
</p><p>    &KIND Si</p><p>
</p><p>      ELEMENT        Si</p><p>
</p><p>      BASIS_SET DZVP-GTH-PADE</p><p>
</p><p>      POTENTIAL GTH-PADE-q4</p><p>
</p><p>    &END KIND</p><p>
</p><p>    &CELL</p><p>
</p><p>      A     5.430697500    0.000000000    0.000000000</p><p>
</p><p>      B     0.000000000    5.430697500    0.000000000</p><p>
</p><p>      C     0.000000000    0.000000000    5.430697500</p><p>
</p><p>      SYMMETRY CUBIC</p><p>
</p><p>      PERIODIC XYZ</p><p>
</p><p>    &END CELL</p><p>
</p><p>    &COORD</p><p>
</p><p>      Si    0.000000000    0.000000000    0.000000000</p><p>
</p><p>      Si    0.000000000    2.715348700    2.715348700</p><p>
</p><p>      Si    2.715348700    2.715348700    0.000000000</p><p>
</p><p>      Si    2.715348700    0.000000000    2.715348700</p><p>
</p><p>      Si    4.073023100    1.357674400    4.073023100</p><p>
</p><p>      Si    1.357674400    1.357674400    1.357674400</p><p>
</p><p>      Si    1.357674400    4.073023100    4.073023100</p><p>
</p><p>      Si    4.073023100    4.073023100    1.357674400</p><p>
</p><p>    &END COORD</p><p>
</p><p>  &END SUBSYS</p><p>
</p><p>  &DFT</p><p>
</p><p>    BASIS_SET_FILE_NAME  BASIS_SET</p><p>
</p><p>    POTENTIAL_FILE_NAME  GTH_POTENTIALS</p><p>
</p><p>    &QS</p><p>
</p><p>      EPS_DEFAULT 1.0E-10</p><p>
</p><p>    &END QS</p><p>
</p><p>    &MGRID</p><p>
</p><p>      NGRIDS 4</p><p>
</p><p>      CUTOFF 300</p><p>
</p><p>      REL_CUTOFF 60</p><p>
</p><p>    &END MGRID</p><p>
</p><p>    &XC</p><p>
</p><p>      &XC_FUNCTIONAL PADE</p><p>
</p><p>      &END XC_FUNCTIONAL</p><p>
</p><p>    &END XC</p><p>
</p><p>    &SCF</p><p>
</p><p>      SCF_GUESS ATOMIC</p><p>
</p><p>      EPS_SCF 1.0E-7</p><p>
</p><p>      MAX_SCF 300</p><p>
</p><p>      ADDED_MOS 10</p><p>
</p><p>      &DIAGONALIZATION  ON</p><p>
</p><p>        ALGORITHM STANDARD</p><p>
</p><p>      &END DIAGONALIZATION</p><p>
</p><p>      &MIXING  T</p><p>
</p><p>        METHOD BROYDEN_MIXING</p><p>
</p><p>        ALPHA 0.4</p><p>
</p><p>        NBROYDEN 8</p><p>
</p><p>      &END MIXING</p><p>
</p><p>      &SMEAR ON</p><p>
</p><p>        METHOD FERMI_DIRAC</p><p>
</p><p>        ELECTRONIC_TEMPERATURE [K] 300</p><p>
</p><p>      &END SMEAR</p><p>
</p><p>    &END SCF</p><p>
</p><p>  &END DFT</p><p>
</p><p>  &PRINT</p><p>
</p><p>    &FORCES ON</p><p>
</p><p>    &END FORCES</p><p>
</p><p>  &END PRINT</p><p>
</p><p>&END FORCE_EVAL</p><p>
</p><p>&MOTION</p><p>
</p><p>  &CELL_OPT</p><p>
</p><p>    KEEP_ANGLES</p><p>
</p><p>    KEEP_SYMMETRY</p><p>
</p><p>  &END CELL_OPT</p><p>
</p><p>  &GEO_OPT</p><p>
</p><p>    TYPE MINIMIZATION</p><p>
</p><p>    MAX_DR    1.0E-03</p><p>
</p><p>    MAX_FORCE 1.0E-03</p><p>
</p><p>    RMS_DR    1.0E-03    </p><p>
</p><p>    RMS_FORCE 1.0E-03</p><p>
</p><p>    MAX_ITER 200</p><p>
</p><p>    OPTIMIZER BFGS</p><p>
</p><p>    !&BFGS</p><p>
</p><p>    !  MAX_STEEP_STEPS  0</p><p>
</p><p>    !  RESTART_LIMIT 9.0E-01</p><p>
</p><p>    !&END BFGS</p><p>
</p><p>  &END GEO_OPT</p><p>
</p><p>  &CONSTRAINT</p><p>
</p><p>    &FIXED_ATOMS</p><p>
</p><p>      COMPONENTS_TO_FIX XYZ</p><p>
</p><p>      LIST 1</p><p>
</p><p>    &END FIXED_ATOMS</p><p>
</p><p>  &END CONSTRAINT</p><p>
</p><p>&END MOTION  </p><p>

</p><p> </p><div>Final cell structure:</div><div><span lang="EN"><p> CELL| Volume [angstrom^3]:                                              173.162</p>
<p> CELL| Vector a [angstrom]:       5.574     0.000     0.000    |a| =       5.574</p>
<p> CELL| Vector b [angstrom]:       0.000     5.574     0.000    |b| =       5.574</p>
<p> CELL| Vector c [angstrom]:       0.000     0.000     5.574    |c| =       5.574</p>
<p> CELL| Angle (b,c), alpha [degree]:                                       90.000</p>
<p> CELL| Angle (a,c), beta  [degree]:                                       90.000</p>
<p> CELL| Angle (a,b), gamma [degree]:                                       90.000</p>
<p> CELL| Requested initial symmetry:                                         CUBIC</p>
<p> CELL| Numerically orthorhombic:                                             YES</p></span></div></span><div> </div><div>Final geometry</div><div><span lang="EN"><p> i =        4, E =       -31.3087094725</p>
<p> Si         0.0000000000        0.0000000000        0.0000000000</p>
<p> Si        -0.0000000224        2.7868973667        2.7868973671</p>
<p> Si         2.7868973677        2.7868973683       -0.0000000226</p>
<p> Si         2.7868973673       -0.0000000224        2.7868973679</p>
<p> Si         4.1803460577        1.3934486673        4.1803460580</p>
<p> Si         1.3934486913        1.3934486907        1.3934486902</p>
<p> Si         1.3934486666        4.1803460575        4.1803460574</p>
<p> Si         4.1803460579        4.1803460580        1.3934486678</p><p> </p><p>Could anyone help me with this confusion? Thanks a lot!</p></span></div><p> </p></span><p> </p></span><p> </p><div>

</div><p> </p></span></div>