Setting up a DFT+U (LDA+U) calculation

You can run CASTEP DFT+U calculations in either the non-spin-polarized mode, when there is no magnetic solution, or in the spin-polarized mode. Most of the correlated systems that require DFT+U treatment exhibit magnetic properties. They are either ferrimagnetic or anti-ferromagnetic, so you must carefully choose the initial spin moments (see Changing atomic spin). In some cases, you must construct a supercell before starting calculations.

The DFT+U formalism is not compatible with NMR, phonon, or polarizability calculations. You cannot use the formalism with mixture atoms or real-space pseudopotentials.

To prepare a DFT+U calculation

Either import a structure from a pre-existing file or construct a new molecule using the sketching tools in the Materials Visualizer.
From the menu bar, select Modify | Electronic Configuration to display the Hubbard U tab of the Electronic Configuration dialog.Set the U values for the orbitals as required.
For more information, see Changing Hubbard U values .
CASTEP provides default effective Hubbard U values as guidelines only, as they are not universal and depend greatly on the chemical environment. The results obtained depend on the value of U as well as the atomic orbitals, so literature values are often inappropriate.
On the Spin tab, specify the Formal spin parameters for your system. Then close the Electronic Configuration dialog.
From the menu bar, choose Modules | CASTEP | Calculation.
Select the Setup tab.
Select a Functional. For more information on functionals, see the theory section.
Select the Use DFT+U checkbox.
Click Run.

The symmetry used in DFT+U calculations is often lower than the crystallographic symmetry, so the number of k-points required increases. Each step performs more calculations, increasing computation times and requiring more steps to achieve convergence.

For cubic systems, converting to tetragonal or rhombohedral symmetry may improve performance.

Sometimes the density mixing scheme may not converge. In such cases, switch to an EDFT minimizer to improve results and choose to Optimize total spin after 1 SCF step.

You can only apply the Hubbard U operator to the states that have an LCAO projector in the pseudopotential. If there is no projector for any orbital with nonzero U value, CASTEP returns the following error message:
Hubbard U is requested for the states which do not have LCAO projectors. Please check HUBBARD_U block in the cell file.
If you think that the states to which you want to apply Hubbard U are really important, use a different pseudopotential (on-the-fly generated potentials are likely to have more projectors than other pseudopotentials).

Setting up a DFT+U (LDA+U) calculation

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