This implies that you would effectively perform a spin-restricted calculation, but with increased computational effort. Note carefully, that using only the keyword unrestricted, without either SpinPolarization or Occupations (or both) will not result in any spin polarization. The actual numbers of spin- \(\alpha\) and spin- \(\beta\) electrons respectively are controlled by the keys SpinPolarization and OCCUPATIONS. The unrestricted mode roughly doubles the computational effort. In the case of Spin-Orbit coupling, it means that Kramer’s symmetry does not have to be satisfied, in which case the key UNRESTRICTED should be used in combination with the subkey SpinOrbitMagnetization NONCOLLINEAR or SpinOrbitMagnetization COLLINEAR of the key Relativity. In an unrestricted calculation spin- \(\alpha\) and spin- \(\beta\) MOs may be spatially different and may have different occupation numbers. However, specification is not meaningful in an unrestricted Spin-Orbit coupled calculation using the (non-)collinear approximation. Specification is only meaningful in a spin-unrestricted calculation. The spin polarization of the system, which is the number of spin-alpha electrons in excess of spin-beta electrons. The program has no knowledge about the existence of materials and will simply try to carry out what you tell it to do. When you specify a particular configuration and/or net charge and/or net spin-polarization of the system, the program will try to compute accordingly, even if the requested configuration has no physical or chemical meaning. It is good practice to always verify which configuration you actually have computed.
In worse cases the program may fail to converge to any state at all.
If SCF convergence is problematic the program may wind up at an excited state, by which (in this context) we mean a one-determinant state with a higher energy than some other one-determinant state with the same net spin polarization. In principle, the program will (by default) attempt to find the lowest-energy spin-restricted (one-determinant) state. If you don’t specify any such keys, certain defaults will apply. The next few keys can be used to specify the electronic configuration.
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