XTB computational module in Chemcraft

 Chemcraft can do some semi-empirical computations (gfn) using an external computational program, xtb. The Windows version of xtb is included in Chemcraft installer together with Chemcraft itself (the author of xtb, Stefan Grimme, allowed us to do that). If you perform a QC computation in Chemcraft, some files are created in "../Chemcraft/Calc/Scratch/"  folder, including a .bat file which runs xtb; when xtb finishes the computation, the output files produced by it are saved into "../Chemcraft/Calc/Scratch/" folder and read and parsed by Chemcraft.

  The Computation menu in Chemcraft has the following menu items:

 - Single point energy                Computes the total energy of current molecule and copies it into Clipboard;

 - Mulliken charges on atoms     Computes the charges on atoms and shows them as labels on atoms;

  - Molecular orbitals                   Computes the MOS;

 - Optimize geometry                 Alters the coordinates of atoms so that the total energy is minimized. Two submenus are available:

    -  Perform computation using Chemcraft optimizer (slow)     With this option, xtb computes only single point energies and gradients at each iteration, while the optimization is performed by Chemcraft. Currently the optimizer in Chemcraft is slow and awkward. One more advantage of this option is that it never finds saddle points instead of minimums (unlike the second menu item);

  When the geometry of your molecule is optimized, the optimization can take a very long time; however in many cases it can be already been almost finished, and you can press "Stop" at any time and then maybe symmetrize your molecule via the "Edit/Set point group" tool.

    -  Perform intrinsic xtb optimization      This menu item runs the xtb optimizer, which is fast, but the optimization process is not shown at the image. If you press Stop, xtb will continue running until the optimization is finished (you can check this if you open files in "/Chemcraft/Cals/Scratch" folder);

 - Geometry optimization + frequencies job      Performs an intrinsic xtb geometry optimization + frequencies computation. Note that since the computed frequencies are shown at the left window, you can recompute them with other isotopes in your molecule;

 - Relaxed PES scan by selected parameter    Before using this menu item, you must select 2-4 atoms which define a distance or an angle or a dihedral in your molecule. You will be able to scan the PES along the selected geometrical parameter. As for the geometry optimization, you can run both Chemcraft optimization (slow and awkward but visualized at each step), and the intrinsic xtb PES scan;

 - Set charge and multiplicity       Here you can specify the charge and the multiplicity of your molecule (0 and 1 by default);

 - Computational settings  Here you can specify the computational method (GFN-1 or GFN-2), and the optimization convergence criterion.

  - Computational settings           Here you can specify the charge and spin (multiplicity) of your molecule, the gfn method (GFN-1 or GFN-2) and the optimization convergence criterion.

  Troubleshooting

  Since the semi-empirical computations are performed not by Chemcraft but by an external program, using this utility has some limitations:

 - If you plan to use this utility, we recommend to install Chemcraft into a hard drive instead of a SDD drive (as far as we know, the SDD drives have limited resource limit);

 - xtb does not recognize paths of files with local characters or special symbols, so you should not install Chemcraft into a folder containing them ("C:\Chemcraft" would be better);

 - Do not run multiple Chemcraft/xtb computations at one computer (this does not mean that you can't run several instances of Chemcraft at one time for other purposes);

 - Sometimes additional problems arise, when xtb does not recognize that the .bat file had ordered to change the default folder to "../Chemcraft/Calc/Scratch/". These problems occur differently with different Windows versions. We found that often the following bug occurs: if you register Chemcraft in Windows as a program for opening e.g. .xyz files, and then click on an .xyz file in Explorer so that this file will be opened by Chemcraft, then try to compute something, the program will crash. However, you will be able to successfully perform the computation with this file, if you close Chemcraft, then run it again, and then press Ctrl+R. Also the computation can be performed, if you open the file via dragging its icon in Windows to Chemcraft icon;

 - Conflicts with antivirus software are also possible.

  Citation

  If you have performed some computations with xtb using the Chemcraft GUI, you can cite the authors as follows:

  The semi-empirical computations were performed using the GFN2-xTB method [1] implemented in xTB program [2], with the Chemcraft program [3] as a graphical user interface for running it.

[1] C. Bannwarth, S. Ehlert and S. Grimme., J. Chem. Theory Comput., 2019,
     15, 1652-1671. DOI: 10.1021/acs.jctc.8b01176

[2] C. Bannwarth, E. Caldeweyher, S. Ehlert, A. Hansen, P. Pracht, J. Seibert, S. Spicher, S. Grimme, Extended tight-binding quantum chemistry methods, WIREs Comput. Mol. Sci., (2021), 11, e1493. DOI: 10.1002/wcms.1493

[3] (See the Chemcraft citation at our website.)

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