CASTEP > Tasks in CASTEP > Setting up CASTEP calculations > Setting up an LDA+U calculation

Setting up an LDA+U calculation

CASTEP LDA+U calculations are only available in the spin-polarized mode, even if there is no magnetic solution. The majority of correlated systems are either ferrimagnetic or anti-ferromagnetic, so the initial spin moments must be carefully chosen (see Changing atomic spin). In some cases, a supercell must be constructed before beginning calculations.

The LDA+U formalism is not compatible with NMR, phonon, or polarizability calculations. The formalism cannot be used with mixture atoms or real-space pseudopotentials.

To set up an LDA+U calculation

  1. Either import a structure from a pre-existing file or construct a new molecule using the sketching tools in the Materials Visualizer.
  2. Select Modify | Electronic Configuration from the menu bar to display the Hubbard U tab of the Electronic Configuration dialog.
  3. Set the U values for the orbitals as desired (see Changing Hubbard U values for further guidance).

    Default effective Hubbard U values should be considered 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.

  4. On the Spin tab set the Formal spin parameters for your system. Then close the Electronic Configuration dialog.
  5. Choose Modules | CASTEP | Calculation from the menu bar.
  6. Select the Setup tab.
  7. Select a Functional from the dropdown list (see the theory section for more information on functionals).
  8. Check the Spin polarized and Use LDA+U checkboxes.
  9. Click the Run button.

The symmetry used in LDA+U calculations is often lower than the crystallographic symmetry, so the number of k-points needed is increased. More calculations are carried out at each step 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 you should switch to an EDFT minimizer to improve results and set the option to Optimize total spin after 1 SCF step.

The Hubbard U operator can be applied only to the states which have a LCAO projector in the pseudopotential. If there is no projector for any orbital with nonzero U value, CASTEP will return 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, you could use a different pseudopotential (on-the-fly generated potentials are likely to have more projectors than other pseudopotentials).

See Also:

Setup tab - CASTEP Calculation dialog
Analyzing CASTEP results

Accelrys Materials Studio 8.0 Help: Wednesday, December 17, 2014
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