5.5 Semiempirical Extended Tight-Binding Calculations

5.5.1 Purpose

tb optimizes a structure using GFN2-xtb as described in the papers of S. Grimme et. al [70, 71].

There are two ways to run xTB calculations:

Either prepare a usual TURBOMOLE input with arbitrary basis set and method. Only the coordinates will be used, all other settings are ignored. If you want to add a charge, please note that you have to use the $tb keyword.
Or just use a simple Cartesian coordinate file (coord) in TURBOMOLE format. No control file is needed. Start a geometry optimization by typing

jobex -level xtb

To add options the control file has to be modified:

$tb 
  charge -1
  gfn <method>
  accuracy 1.0
  etemp 300
  broydamp 
  maxiter <number>

The options are:

charge 0: optionally sets the molecular total charge. If not specified, a neutral input is assumed.
gfn 2: choose method, GFN(1)-xTB or GFN2-xTB, default is 2
accuracy 1.0: accuracy for SCC calculations, lower value means higher accuracy. Default is 1.0
etemp 300: electronic temperature in Kelvin, default is 300K
broydamp 0.4: Broyden damping for charge mixing, default is 0.4
maxiter 250: maximum number of SCC iterations, default is 250