Manual

xtb

This keyword controls the xTB calculation options.

Options

chrg

This option is used to calculate the total charge of the system.

Value	An integer
Default	`0`

uhf

Define the number of unpaired electrons.

Value	An integer
Default	`0`

gfn

Define the xTB Hamiltonian type.

Value	`1` The GFN1-xTB Hamiltonian.
Value	`2` The GFN2-xTB Hamiltonian.
Default	`2`

gbsa

The solvent name. If it is not given, the xTB is calculated in the gas phase. Otherwise, the generalized Born model will be used to consider solvent effects.

Value	`H2O`
	`Acetone`
	`Acetonitrile`
	`Benzene`
	`CH2Cl2`
	`CHCl3`
	`CS2`
	`DMSO`
	`Ether`
	`Hexane`
	`Toluene`
	`THF`
Default	None

Theoretical Background

xTB is a powerful semiempirical quantum chemical method proposed by Stefan Grimme. It has been designed for the purpose of facilitating rapid and accurate electronic structure calculations of large molecular systems (approximately 1000 atoms). It can be used to rapidly give a good initial guess for DFT calculations.

xTB has several Hamiltonians. Qbics supports GFN-1 and GFN-2. It seems that:

GFN1-xTB: When a system contains metal cluster-like structures, this is better.
GFN2-xTB: For most organic molecules with no or a few metal atoms.

To consider solvent effects, one can use gbsa.

xTB can also work with CHARMM force field, i.e., xTB/MM method. This is a powerful way to study large, complicated systems.

For all features of the xTB method, please refer to the xTB manual.

Input Examples

Example: Geometry Optimization by GFN2-xTB for Cantharidin

Perform geometry optimization by GFN2-xTB for an organic molecule, cantharidin:

xtb
    chrg 0
    gfn  2  # GFN2-xTB.
end

# Cantharidin
mol
    C    1.39900     0.71500     1.06800
    C    1.92100    -0.06900    -0.14800
    C    0.78600    -0.69800    -1.04400
    C    0.13800    -2.16200    -0.87800
    C    1.29700    -3.05700    -0.50500
    C    2.44700    -2.21000    -1.00400
    O    1.70700    -1.35300    -1.91200
    C    2.98300    -1.16300    -0.03500
    C    3.42500    -1.73000     1.31300
    C    4.18700    -0.54700    -0.69400
    O    5.29000    -1.08800    -0.78200
    O    3.91700     0.65900    -1.20900
    C    2.61300     0.98300    -0.99400
    O    2.10200     2.01700    -1.40000
    H    0.94400     0.07300     1.82400
    H    0.61300     1.41400     0.73300
    H    2.16400     1.34500     1.53700
    H    0.05600    -0.55500    -0.22800
    H   -0.28600    -2.46400    -1.85300
    H   -0.70400    -2.25200    -0.17500
    H    1.22700    -4.00600    -1.04600
    H    1.31500    -3.28300     0.56100
    H    3.20900    -2.78100    -1.54000
    H    2.64400    -2.28900     1.83300
    H    3.79300    -0.95400     1.99200
    H    4.26100    -2.43200     1.16600
end

task
    opt xtb
end

Example: Geometry Optimization by GFN1-xTB for Co₆Te₈(P(Et)₃)₆

Perform geometry optimization by GFN1-xTB for a quantum dot Co₆Te₈(P(Et)₃)₆. For such inorganic systems, GFN2-xTB may not converge.

xtb
    chrg 0
    gfn  1  # GFN1-xTB. Do you want to try GFN2-xTB?
end

# Quantum dot: Co6Te8(P(Et)3)6
mol
    Co     0.07482     0.19972     2.25274
    Co    -0.57236     2.10842    -0.12184
    Co     2.14275     0.01139    -0.00690
    Co     0.05260    -2.18506    -0.00075
    Co    -0.03312     0.01926    -2.21965
    Co    -2.27062    -0.26857     0.13243
    Te    -1.92498     1.48104     1.75344
    Te     1.51638     1.80079    -1.53651
    Te    -1.62601    -1.71313    -1.72073
    Te    -1.37710    -1.68992     1.91428
    Te     1.61324    -1.60191     1.77284
    Te    -2.00357     1.32469    -1.66240
    Te     1.66650    -1.63285    -1.74916
    Te     1.57938     1.94121     1.38532
    P     -4.02850    -1.49064     0.26273
    C     -5.19829    -1.41084    -1.21441
    H     -4.71377    -1.92702    -2.04886
    H     -6.13262    -1.93467    -0.99467
    C     -5.52751     0.01728    -1.64482
    H     -6.14156     0.00324    -2.54679
    H     -6.07317     0.54663    -0.86250
    H     -4.61066     0.57223    -1.85793
    C     -5.30003    -1.10968     1.60881
    H     -5.85560    -0.21489     1.31212
    H     -6.02467    -1.92631     1.68168
    C     -4.67909    -0.85880     2.97985
    H     -5.45790    -0.66760     3.72007
    H     -4.08936    -1.71732     3.30379
    H     -4.01919     0.01101     2.93660
    C     -3.88381    -3.34584     0.48895
    H     -3.69815    -3.52421     1.55200
    H     -2.98357    -3.67170    -0.04002
    C     -5.05361    -4.21895     0.03691
    H     -4.86097    -5.25352     0.32700
    H     -5.98965    -3.90442     0.49944
    H     -5.17056    -4.18666    -1.04614
    P      4.26310    -0.30746     0.15884
    C      5.20849     0.27270    -1.36436
    H      4.75167     1.21294    -1.69163
    H      6.25460     0.48490    -1.12936
    C      5.15727    -0.72993    -2.51672
    H      5.46512    -0.25078    -3.44736
    H      5.81933    -1.57520    -2.32685
    H      4.14177    -1.11524    -2.64233
    C      5.11814    -1.95972     0.40905
    H      4.65430    -2.67193    -0.28084
    H      6.17155    -1.88201     0.12313
    C      5.02841    -2.52709     1.82421
    H      5.41753    -3.54657     1.84052
    H      5.60186    -1.92517     2.52917
    H      3.98817    -2.54672     2.15600
    C      5.07249     0.73925     1.49299
    H      4.67423     0.39189     2.45186
    H      4.69613     1.75999     1.36590
    C      6.59882     0.77674     1.57721
    H      6.89665     1.35016     2.45694
    H      7.01872    -0.22476     1.66541
    H      7.03038     1.25623     0.69893
    P      0.54020     0.66347    -4.18625
    C      0.22564     2.40032    -4.82496
    H      0.71085     3.09306    -4.13064
    H      0.69956     2.53727    -5.80137
    C     -1.25205     2.77005    -4.94545
    H     -1.35213     3.83547    -5.15929
    H     -1.73005     2.21033    -5.75002
    H     -1.77854     2.55103    -4.01396
    C     -0.31031    -0.32739    -5.55031
    H     -1.38586    -0.13635    -5.48194
    H      0.01101     0.00196    -6.54210
    C     -0.07813    -1.83035    -5.40257
    H     -0.63237    -2.37433    -6.16909
    H      0.97995    -2.07651    -5.49904
    H     -0.42183    -2.16791    -4.42187
    C      2.34305     0.47110    -4.66339
    H      2.65871    -0.51529    -4.31013
    H      2.90238     1.20335    -4.07212
    C      2.73771     0.61134    -6.13325
    H      3.82165     0.51779    -6.22187
    H      2.28001    -0.16835    -6.74157
    H      2.44775     1.58156    -6.53622
    P      0.62497     1.10725     4.12138
    C      2.39189     1.12488     4.75495
    H      2.99045     1.67998     4.02630
    H      2.45236     1.66544     5.70425
    C      2.99908    -0.26481     4.94187
    H      4.06777    -0.18045     5.14607
    H      2.52854    -0.78692     5.77566
    H      2.86297    -0.86705     4.04081
    C     -0.20941     0.28666     5.60490
    H      0.15369    -0.74420     5.66370
    H      0.07196     0.77828     6.54013
    C     -1.73060     0.25398     5.47187
    H     -2.17355    -0.26257     6.32492
    H     -2.14306     1.26238     5.42400
    H     -2.01420    -0.27566     4.55989
    C      0.18744     2.91028     4.39005
    H     -0.80673     3.06756     3.95946
    H      0.88329     3.49228     3.77726
    C      0.19716     3.46228     5.81507
    H      0.01322     4.53785     5.78327
    H     -0.58383     3.00325     6.42068
    H      1.15764     3.29671     6.30321
    P      0.50037    -4.23165    -0.45649
    C      0.33638    -4.95275    -2.18024
    H      0.88223    -4.28949    -2.85878
    H      0.82383    -5.93134    -2.22239
    C     -1.09935    -5.10086    -2.67831
    H     -1.09940    -5.40811    -3.72540
    H     -1.64199    -5.84739    -2.09777
    H     -1.62712    -4.14741    -2.59715
    C     -0.42694    -5.55682     0.51882
    H     -1.48401    -5.50017     0.24082
    H     -0.08115    -6.55916     0.25173
    C     -0.31641    -5.35550     2.02886
    H     -0.92092    -6.09745     2.55307
    H      0.71702    -5.44947     2.36437
    H     -0.67723    -4.36005     2.30237
    C      2.28540    -4.69974    -0.10975
    H      2.56512    -4.22430     0.83575
    H      2.88463    -4.21217    -0.88607
    C      2.66107    -6.17949    -0.04535
    H      3.74212    -6.26881     0.07682
    H      2.18351    -6.67142     0.80162
    H      2.37807    -6.70546    -0.95721
    P     -1.88362     3.80220     0.05043
    C     -3.72133     3.80113     0.43439
    H     -3.92611     2.89184     1.00638
    H     -3.95713     4.64298     1.09168
    C     -4.66286     3.84410    -0.76765
    H     -5.68683     3.66404    -0.43496
    H     -4.62838     4.81311    -1.26529
    H     -4.39293     3.06880    -1.48575
    C     -1.85215     4.95783    -1.44558
    H     -2.32560     4.43432    -2.28107
    H     -2.43731     5.86170    -1.25698
    C     -0.43388     5.34449    -1.86016
    H     -0.45831     5.92448    -2.78423
    H      0.05377     5.94199    -1.08916
    H      0.16816     4.44750    -2.03047
    C     -1.28938     4.97422     1.41176
    H     -0.19434     4.95693     1.40891
    H     -1.60040     4.51594     2.35618
    C     -1.76235     6.42709     1.38328
    H     -1.44247     6.92890     2.29841
    H     -1.33556     6.96411     0.53662
    H     -2.84893     6.49209     1.32206
end

task
    opt xtb
end

Example: Geometry Optimization by GFN2-xTB for Avibactam in Implicit Solvent

Perform geometry optimization by GFN2-xTB for an organic anion in implicit DMSO solvent.

xtb
    chrg -1 # The charge is set here.
    gfn   2
    gbsa  DMSO # This is in the list of xtb/gbsa item.
end

# Avibactam anion.
mol
    N     1.83500     1.56700     0.16500
    C     2.12100     0.23900     0.16900
    O     1.81100    -0.44100    -0.81200
    C     2.91300    -0.26000     1.37900
    C     3.39800    -1.72600     1.24400
    C     3.05700    -2.71300     2.38100
    C     2.33800    -2.11500     3.59200
    C     1.27000    -1.15400     3.10000
    C     2.15100     0.03200     2.68300
    C     3.19600     0.07600     3.78800
    O     3.93800     1.02100     3.98300
    N     3.17400    -1.18300     4.31100
    O     4.05400    -1.65700     5.28400
    S     3.38300    -1.70500     6.79900
    O     4.42700    -2.31500     7.61600
    O     3.08700    -0.30900     7.10900
    O     2.19100    -2.53900     6.62800
    H     1.26500     1.98900    -0.55600
    H     2.09600     2.17600     0.93800
    H     3.82300     0.36100     1.36100
    H     4.49300    -1.70100     1.15700
    H     3.06500    -2.17300     0.29900
    H     3.97500    -3.21900     2.70500
    H     2.41300    -3.49600     1.96000
    H     1.94500    -2.89800     4.24100
    H     0.61200    -0.86000     3.92800
    H     0.65100    -1.55100     2.29000
    H     1.57300     0.96000     2.68200
end

task
    opt xtb
end

Example: Geometry Optimization for Organic Radical C₁₃H₉

Perform geometry optimization by GFN2-xTB for an organic radical C₁₃H₉.

xtb
    chrg  0
    gfn   2
    uhf   1 # The number of unpaired electrons is 1.
end

# C13H9
mol
    C  0.   -1.2266   -2.135
    C  0.    0.0000   -1.424
    C  0.    1.2266   -2.135
    C  0.   -2.4574   -1.388
    C  0.    0.0000    0.008
    C  0.   -1.2295    0.717
    C  0.   -2.4588   -0.032
    C  0.   -1.2128    2.117
    H  0.   -2.1632    2.672
    C  0.    0.0000    2.807
    C  0.    1.2128    2.117
    C  0.    1.2295    0.717
    C  0.    2.4588   -0.032
    C  0.    2.4574   -1.388
    H  0.    3.4035   -1.951
    H  0.    3.4010    0.542
    H  0.   -3.4035   -1.951
    H  0.   -3.4010    0.542
    H  0.    0.0000    3.908
    H  0.    2.1632    2.672
    H  0.   -1.2130   -3.109
    H  0.    1.2130   -3.109
end

task
    opt xtb
end