LCHIMAG

LCHIMAG = .TRUE. | .FALSE.
Default: LCHIMAG = .FALSE. 

Description: calculate the chemical shifts by means of linear response.

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For LCHIMAG=.TRUE., VASP calculates the chemical shift tensors by means of linear response.

The chemical shift tensor is defined as:

$\sigma_{\mathbf{R}ij} = \frac{ \partial B^{\mathrm{ind}}(\mathbf{R})_i}{ \partial B^{\mathrm{ext}}_j}$

Here R denotes the atomic nuclear site, i and j denote cartesian indices, B^ext an applied DC external magnetic field and B^ind(R) the induced magnetic field at the nucleus. NMR experiments yield information on the symmetric part of the tensor. VASP can calculate chemical shifts for crystalline systems using the linear response method of Yates, Pickard and Mauri.

A typical INCAR could look like this:

PREC = A               # nice
ENCUT = 600.0          # typically higher cutoffs than usual are needed
EDIFF = 1E-8           # you need much smaller EDIFFs than normal.
ISMEAR = 0; SIGMA= 0.1 # no fancy smearings, SIGMA sufficiently small
LREAL = A              # helps for speed for large systems, not necessary per se

LCHIMAG = .TRUE.       # to switch on linear response for chemical shifts
DQ = 0.001             # often the default is sufficient
ICHIBARE = 1           # often the default is sufficient
LNMR_SYM_RED = .TRUE.  # be on the safe side
NSLPLINE = .TRUE.      # only needed if LREAL is NOT set.

The first block of tags in the INCAR above expresses the fact that the calculations of chemical shifts by means of linear response often require a high accuracy (PREC=A, EDIFF≤1E-8, high ENCUT).

The chemical shifts are calculated from the orbital magnetic response under the assumption that the system is an insulator. It makes no sense to use smearing schemes intended for metals, indeed, doing so can generate nonsense. It is safe to use ISMEAR=0 and make SIGMA so small that states have no fractional occupancies.

The seçond block of tags switches on the calculation of the chemical shifts (LCHIMAG=.TRUE.), and controls several aspects of the finite difference k-space derivatives (Eqs. 38, 40, and 47 in the work of Yates et al.):

• DQ is the step size for the finite difference k-space derivative. Typical values are in the range [0.001 - 0.003]. The default is often sufficient.

• ICHIBARE is the order of the finite difference stencil used to calculate the magnetic susceptibility (second order derivative in Eq. 47 of Yates et al.). ICHIBARE may be set to 1, 2, or 3. Often the default (ICHIBARE=1) is sufficient. A higher ICHIBARE results in a substantial increase of the computational load.

• For NLSPLINE=.TRUE., the PAW projectors in reciprocal space (LREAL=.FALSE.) are set up using a spline interpolation so that they are k-differentiable. This only slightly affects the chemical shifts themselves, but can have impact on the susceptibility contribution (the aforementioned Eq. 47). It is advised to set NLSPLINE=.TRUE., but only in case of calculation of chemical shift. As this option also gives slightly different total energies, it is advised to use the default NLSPLINE=.FALSE. for compatibility in all other calculations. Real space projectors are k-differentiable by construction.

• The star on which the k-space derivative is calculated is oriented along the cartesian directions in k-space. If the symmetry operations in k-space do not map this star onto itself, erroneous results can be obtained. To have VASP check for such operations, set LNMR_SYM_RED=.TRUE., and such operations will be discarded, resulting in a larger IBZ. In case of any doubt set LNMR_SYM_RED=.TRUE. Beware: It matters how the real space lattice vectors are set up relative to the cartesian coordinates in POSCAR. It determines the orientation of the k-space star and hence can affect the efficiency via the number of k-points in the IBZ.

Related Tags and Sections

DQ, ICHIBARE, LNMR_SYM_RED, NLSPLINE

References

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