Periodic Calculations

Starting from xtb version 6.7.0, the GFN-FF method supports periodic calculations using periodic boundary conditions. Additionally, a special run mode called mcGFN-FF, which includes scaled non-covalent interactions (NCI), was introduced and is recommended for periodic systems. This mode can be used for a fast optimization of periodic systems, optionally without optimizing the cell parameters (–nocellopt).

A periodic GFN-FF calculation is invoked similarly to a normal GFN-FF calculation, with the exception that the structure input file must contain lattice information. Therefore, only the Turbomole, Vasp’s POSCAR/CONTCAR, and genFormat file formats are supported.

Note: With xtb, only GFN-FF is available for periodic calculations and no Hessian computation is possible. However, in DFTB+, a periodic implementation of the GFN-xTB methods is available.

To perform a periodic optimization with mcGFN-FF, use the following command:

xtb struc.coord --mcgfnff --opt

The resulting structure can be found in the xtbopt.coord file, which includes the optimized lattice parameters. A trajectory of the optimization is available in the xtboptlog.cif file.

Exercise

As an example, optimize the unit cell of benzene. To visualize the structure, you can convert the Turbomole format to xyz using a tool like mctc-convert and then open it with a graphical program such as Molden.

$coord
    7.10986726811546E+00    1.39803317039751E+01    9.79695027924021E+00      C
    7.11375821259805E+00    8.76795203096078E+00    3.36407395831929E+00      C
    8.60482372324771E+00    5.13823133707231E+00    1.25907033747429E+01      C
    9.75112376027930E+00    7.53368379774428E+00    1.25988946977277E+01      C
    5.96704128865761E+00    6.37267695865231E+00    3.35549866706958E+00      C
    9.00267279659253E+00    9.34854414468852E+00    1.58886068020756E+00      C
    6.71563121464733E+00    4.55781661170807E+00    1.56671851026428E+00      C
    2.01592612889045E+00    1.34582777502016E+01    1.56454269009644E+00      C
    1.36276161633625E+01    5.11410651963663E-01    1.59001258500230E+00      C
    2.76479397948607E+00    1.52711868251473E+01    3.35524268822630E+00      C
    1.28790262373728E+01    1.64375153151443E+01    1.25977427929330E+01      C
    1.62071733930167E+00    5.14124693925177E+00    9.79797419461331E+00      C
    1.27150507198922E-01    8.76741986587029E+00    6.22310165885711E+00      C
    2.76687841403031E+00    7.53687678828724E+00    9.78696710435249E+00      C
    1.28772197274344E+01    6.37179001683482E+00    6.23423673853957E+00      C
    2.01689886501110E+00    9.34978586323301E+00    7.99677906390919E+00      C
    1.27706356410721E-01    1.40413533010183E+01    1.25881435863102E+01      C
    1.61946667857512E+00    1.76671714509098E+01    3.36548184195730E+00      C
    8.99919873901879E+00    1.33969013764314E+01    8.02314488476650E+00      C
    5.96523477871927E+00    1.63766711064646E+01    9.78632715724430E+00      C
    9.74959517494686E+00    1.52094556746502E+01    6.23270086547992E+00      C
    8.60482372324771E+00    1.76057950771396E+01    6.22182176464074E+00      C
    6.71535329004143E+00    4.50389054920479E-01    7.99562715911445E+00      C
    1.36270603141507E+01    4.55888094188905E+00    8.02442477898288E+00      C
    1.12337125704490E+01    7.97130089049293E+00    1.12071377268419E+01      H
    5.79806312827085E+00    2.31828852255051E+00    7.97514885165245E+00      H
    9.14344160948052E+00    3.72249480799774E+00    1.11681009532424E+01      H
    6.57555721327409E+00    1.01835111716719E+01    4.78718833750633E+00      H
    4.24696590274695E+00    1.48321506254907E+01    4.74712764853380E+00      H
    1.27099091146831E+01    2.37913273123019E+00    1.60972295593447E+00      H
    2.93391110217578E+00    1.15907330592985E+01    1.54457634032100E+00      H
    1.13967153518090E+01    1.68763741264374E+01    1.12061138114688E+01      H
    4.48473040309378E+00    5.93505986590366E+00    4.74751161679871E+00      H
    2.15975211244333E+00    3.72728429381217E+00    1.12222404785951E+01      H
    5.79667350524135E+00    2.69080408589553E+00    1.54803205470521E+00      H
    9.92149154369556E+00    1.12155566705011E+01    1.60729115692336E+00      H
    1.34851798028391E+01    1.26270358788517E+01    1.11638773023284E+01      H
    1.13944919549618E+01    5.93293120554168E+00    4.84311971476190E+00      H
    4.25002307341184E+00    7.97555821121688E+00    1.11778281492869E+01      H
    1.27082415670477E+01    2.69186841607651E+00    8.04490308644487E+00      H
    2.93557864981118E+00    1.12169757774090E+01    7.97591678818228E+00      H
    6.56985975885316E+00    1.25663690585355E+01    1.12210885738004E+01      H
    9.14510915711592E+00    1.28092137281638E+00    4.79793944892387E+00      H
    1.12321839851166E+01    1.47700646982665E+01    4.84171183112389E+00      H
    4.48153427012594E+00    1.68157073061212E+01    1.11765482550705E+01      H
    9.91718371230412E+00    1.15293566855284E+01    8.04375118165014E+00      H
    1.34839291421126E+01    1.01815598996734E+01    4.79909135371861E+00      H
    2.15794560250498E+00    1.34247513495005E+00    4.79000410478235E+00      H
$periodic 3
$lattice bohr
    13.89623029496416    0.00000000000000    0.00000000000000
     0.00000000000000   17.73883634976286    0.00000000000000
     0.00000000000000    0.00000000000000   12.79894216374709
$end