Nuclear Quadrupole Interaction (EFG)
************************************

   QTENS

This key activates the computation of the Nuclear Electric Quadrupole
Hyperfine interaction. It can be applied to both open-shell and
closed-shell systems. QTENS yields the nuclear electric quadrupole
hyperfine interaction (Q-tensor) [1], which is directly related to the
Electric Field Gradient (EFG). The Q-tensor elements (in MHz) equal
the electric field gradient tensor elements (in a.u.) times 234.9647
times the nuclear quadrupole moment (NQM in barn units, 1 barn =
10-28m^2  = 10-24cm^2 ) and divided by 2I(2I-1), where I is the
nuclear spin. The Nuclear Quadrupole Coupling Constant (NQCC) (in MHz)
is the principal component with the largest absolute magnitude of the
EFG (in a.u.) times 234.9647 times the nuclear quadrupole moment (in
barn units). The electric field gradient tensor is printed next to the
Q-tensor.

In the case of ZORA, the program will calculate the EFG in the so-
called ZORA-4 approximation, which includes a small component density
("picture-change correction"), see [1]. If you also want to calculate
the EFG using the ZORA density, that is, in the Foldy-Wouthuysen
picture, include the keyword:

   PRINT FWPICTURE

If QTENS is used for ^57Fe, ^119Sn, ^125Te, ^193Ir, and ^197Au,
quadrupole splittings are written in units of mm/s, used in Mössbauer
spectroscopy.

**Analysis of the EFG**

With the EFG keyword in AOResponse, a Mulliken type analysis of the
EFG principal components, and an analysis in terms of canonical MOs,
can be performed. Required is symmetry NOSYM. This analysis is not
implemented for spin-orbit coupling. For an NBO analysis of the EFG,
see the section on NBO analysis. For an explanation of the output and
a general usage tutorial, see [2]. For further references and
recommended citations, see [3].

   Symmetry NOSYM
   Aoresponse
    efg NUC
   end

"efg NUC"
   Here NUC is the index of the nucleus at which the EFG is to be
   computed (ADF internal atom ordering). Available for one nucleus at
   a time.

[1] E. van Lenthe and E.J. Baerends, *Density functional calculations
    of nuclear quadrupole coupling constants in the zero-order regular
    approximation for relativistic effects*, Journal of Chemical
    Physics 112, 8279 (2000)

[2] J. Autschbach, S. Zheng, and R.W. Schurko, *Analysis of Electric
    Field Gradient Tensors at Quadrupolar Nuclei in Common Structural
    Motifs*, Concepts in Magnetic Resonance Part A 36A, 84 (2010)

[3] A.J. Rossini, R.W. Mills, G.A. Briscoe, E.L. Norton, S.J. Geier,
    I. Hung, S. Zheng, J. Autschbach, and R.W. Schurko, *Solid-State
    Chlorine NMR of Group IV Transition Metal Organometallic
    Complexes*, Journal of the American Chemical Society 131, 3317
    (2009)
