Within the electric field linear response formalism, CASTEP enables you to calculate optical (ω =
∞) and dc (ω = 0) dielectric permittivity or optical (ω = ∞) and static (ω = 0) molecular polarizability. These
properties are calculated when the TASK keyword in the input parameters
.param) file is set to either Efield or
Phonon+Efield. In the latter case, infrared intensities (response to an electric field in the infrared range) are
calculated as well if there is a Γ-point among the phonon k-points.
The calculated results are somewhat sensitive to the value of the scissors correction. This correction amounts to the difference between the calculated and experimental band gap. It is possible to estimate this correction when the experimental data are available by performing the band structure run with CASTEP and comparing the calculated band gap with the experimental value.
To calculate polarizability, IR and Raman spectra
CASTEP can be used to calculate the electric field response of isolated molecules as well as of solids. It is recommended that only the Γ-point is used for electronic calculations on molecular systems. In addition, the all Bands/EDFT electronic minimizer is known to be more efficient for studies of isolated molecules in supercell geometry than the density mixing minimizer. Therefore, it is recommended that you change the Electronic minimizer setting to All Bands/EDFT on the SCF tab of the CASTEP Electronic Options dialog when performing such calculations.
Calculations for "molecule in a box" systems that do not require geometry optimization can be sped up if the molecular symmetry is utilized. Use the Find Symmetry tool to find and apply the symmetry of the molecule to the supercell.
It is important to select the correct description of the system from the System type dropdown list when calculating the electric field response for a molecular system. The wrong system description can significantly affect the molecular polarizability values obtained.
The current version of CASTEP has a number of limitations related to electric field response calculations; they are essentially the same as those for phonon calculations. The only settings that are supported for electric field response calculations are the following:
It is not sufficient to use fixed orbital occupancies (i.e. by checking the Fix occupancy checkbox on the SCF tab of the CASTEP Electronic Options dialog) if the system under investigation should be nonmetallic according to DFT results from a non-spin-polarized calculation. Attempting to study a metallic system using fixed orbital occupancies will result in poor convergence of the electric field response calculation; the symptoms of which will very large numbers for IR and Raman intensities and for polarizability in the output file. If this is the case, check whether your system is metallic by running either density of states or band structure calculations.
If any of these restrictions is violated by the current settings, an error message will be displayed with an explanation of the problem. The offending setting must be changed in order to proceed with the calculation.
Gradient-corrected exchange-correlation functionals are supported in linear response calculations, although the results may be less accurate than those obtained with LDA and the computational cost is noticeably higher.
Requesting vibrational properties
Setting up a calculation on an isolated molecule
CASTEP Calculation dialog