.. _dipro: ------------------- Spin-polarization ------------------- This guide aims to give a general overview of spin-polarized (sp)GFNn-xTB calculations within the ``xtb`` program package. .. note:: This feature is available in ``xtb`` in version 6.6.1 and upward via the tblite. .. contents:: General description =================== GFN1-xTB and GFN2-xTB both treat open-shell systems in an restricted open-shell fashion since their energy expressions are spin-independent. Therefore, higher electron spin multiplicities will always be higher in energy than the low-spin multiplicities. However, the methods are occupation dependent and the geometries of high-spin states can still be optimized. spGFNn-xTB =========== To remedy this issue the spin-polarized GFN1 and GFN2-xTB methods (spGFNn-xTB) have been developed and are available in ``xtb`` via the tblite library with the call: .. code:: sh > xtb --spinpol --tblite .. note:: The spin polarization will only affect the energies of open-shell configurations and not of singlets Example =================== As example we will calculate the spin splittings of a mixed-valence **FeCo** complex with spGFN2-xTB to find the electronic ground state: .. tab-set:: .. tab-item:: FeCO .. figure:: ../figures/spgfn.png .. tab-item:: mol.xyz .. code-block:: none 96 Co 4.2981 7.8875 -2.4539 Fe 4.2936 7.6860 -0.2490 N 4.3177 8.2394 -4.7611 N 2.5367 8.7591 -2.6654 N 2.3093 8.5146 -0.4585 C 2.8977 8.4151 -5.0518 H 2.4313 7.4287 -5.0575 H 2.7341 8.8807 -6.0379 C 2.1870 9.2538 -3.9732 H 2.4985 10.3023 -4.0724 H 1.1027 9.2137 -4.1534 C 1.8587 9.0585 -1.5831 C 0.5926 9.8363 -1.6028 C -0.5637 9.2492 -1.0959 H -0.5146 8.2451 -0.7007 C -1.7596 9.9421 -1.0971 H -2.6509 9.4763 -0.7029 C -1.8155 11.2325 -1.5975 H -2.7493 11.7751 -1.5960 C -0.6674 11.8282 -2.0926 H -0.7045 12.8389 -2.4730 C 0.5294 11.1352 -2.0985 H 1.4270 11.6140 -2.4622 C 1.9785 8.9213 0.8129 C 1.6789 10.2428 1.1731 H 1.6533 11.0098 0.4144 C 1.4548 10.5742 2.4929 H 1.2329 11.6001 2.7502 C 1.5192 9.6137 3.4891 H 1.3431 9.8845 4.5191 C 1.8213 8.3036 3.1506 H 1.8771 7.5470 3.9204 C 2.0494 7.9614 1.8355 H 2.2654 6.9364 1.5551 C 5.3288 9.1918 1.9786 H 5.5540 8.1404 1.7923 H 5.1441 8.9630 4.0893 N 5.5471 9.4758 -2.5880 N 5.4239 9.4980 -0.3630 C 5.1066 9.4430 -4.9974 H 4.4198 10.2884 -5.0609 H 5.6486 9.3764 -5.9534 C 6.1233 9.7533 -3.8739 H 7.0099 9.1183 -4.0024 H 6.4521 10.7963 -3.9819 C 5.8484 10.0894 -1.4741 C 6.6871 11.3166 -1.4253 C 6.2995 12.4761 -2.0920 H 5.3646 12.4913 -2.6351 C 7.0867 13.6116 -2.0320 H 6.7762 14.5093 -2.5474 C 8.2677 13.6012 -1.3084 H 8.8808 14.4891 -1.2624 C 8.6554 12.4529 -0.6380 H 9.5718 12.4449 -0.0664 C 7.8701 11.3159 -0.6942 H 8.1642 10.4190 -0.1694 C 5.2950 10.0694 0.8789 C 5.0337 11.4236 1.1229 H 4.9714 12.1140 0.2955 C 4.8255 11.8729 2.4115 H 4.6221 12.9211 2.5798 C 4.8647 10.9989 3.4848 H 4.6959 11.3605 4.4876 C 5.1147 9.6533 3.2589 N 5.5409 5.9316 -0.8551 C 6.9688 4.5777 -2.3437 N 5.5363 6.4511 -3.0183 H 6.1674 5.3706 -4.7257 H 6.9044 6.9670 -4.5066 C 4.8934 7.0165 -5.3047 C 5.9719 6.3998 -4.3893 C 6.0308 5.6968 -2.0668 H 4.0923 6.2814 -5.4031 H 5.3284 7.1836 -6.3039 C 6.0102 5.4138 0.3274 C 7.6940 4.5970 1.8539 C 5.0873 5.3600 1.3878 C 5.4599 4.9302 2.6424 C 6.7691 4.5391 2.8839 C 7.3321 5.0289 0.5932 H 8.0785 5.0923 -0.1832 H 8.7188 4.3056 2.0376 H 4.7300 4.8956 3.4388 H 7.0654 4.1993 3.8647 H 4.0598 5.6485 1.1710 C 6.5768 3.2760 -2.0424 C 8.6681 2.4371 -2.8654 C 8.2226 4.7949 -2.9076 H 7.1084 1.2072 -2.0670 H 5.6060 3.1067 -1.6000 C 9.0673 3.7296 -3.1624 C 7.4214 2.2136 -2.3040 H 8.5451 5.8037 -3.1213 H 10.0420 3.9086 -3.5932 H 9.3287 1.6068 -3.0670 .. code:: bash > xtb mol.xyz --spinpol --tblite --uhf with being the number of unparied electrons 0, 2, 4, 6, and 8 (with the correspond multiplicities 1, 3, 5, 7, and 9). The ground state is experimentally known to be a septet. The total energies in Hartree are shown below: +--------------+----------------------+---------------------+ | Multiplicity | E(GFN2-xTB) | E(spGFN2-xTB) | +==============+======================+=====================+ | 1 | -145.1025 | -145.1025 | +--------------+----------------------+---------------------+ | 3 | -145.0996 | -145.1185 | +--------------+----------------------+---------------------+ | 5 | -145.0870 | -145.1712 | +--------------+----------------------+---------------------+ | 7 | -145.0574 | -145.1904 | +--------------+----------------------+---------------------+ | 9 | -144.9653 | -145.1209 | +--------------+----------------------+---------------------+ and the spin-splittings with respect to the septet ground-state in kcal/mol are: +--------------+----------------------+---------------------+ | Multiplicity | ΔE(GFN2-xTB) | ΔE(spGFN2-xTB) | +==============+======================+=====================+ | 1 | -28.3 | 55.2 | +--------------+----------------------+---------------------+ | 3 | -26.4 | 45.2 | +--------------+----------------------+---------------------+ | 5 | -18.5 | 12.1 | +--------------+----------------------+---------------------+ | 7 | 0.0 | 0.0 | +--------------+----------------------+---------------------+ | 9 | 57.8 | 43.6 | +--------------+----------------------+---------------------+ The correct gound state can be found with spGFN2-xTB but not with GFN2-xTB. Please find more information on spGFNn-xTB methods in our publication: H. Neugebauer, B. Bädorf, S. Ehlert, A. Hansen, S. Grimme, *J. Comput. Chem.*, **2023**, 44(27), 2120. `DOI: 10.1002/jcc.27185 `_