NMR shifts with relativistic DFT
********************************

ADF Spectroscopy Nuclear magnetic resonance (NMR) Relativistic DFT
Spin-orbit coupling ZORA

See also the FAQ on recommend settings for NMR calculations..


Scalar relativistic and spin-orbit coupled calculations of NMR chemical shifts
==============================================================================

Scalar relativity and spin-orbit coupling can have a large effect on
the calculated NMR parameters, especially for heavy atoms.
Relativistic effects can also be significant for a light element that
is close to a heavy atom. This tutorial addresses how the NMR chemical
shift of ^1H in the hydrogen halides series (HF, HCl, HBr, HI) is
affected by spin-orbit coupling. For more information on how
relativity effects the calculation of NMR parameters, see Jochen
Autschbach‘s chapter on 'Relativistic Effects on NMR Parameters' in
High Resolution NMR Spectroscopy edited by Rubén Contreras.

In this tutorial, we compare the NMR results with the following two XC
functionals: **GGA PBE** and **hybrid PBE0** (recommended). Thus in
total 16 calculations will be performed: 4 (molecules HF, HCl, HBr,
and HI) times 2 (PBE and PBE0) times 2 (scalar relativistic and spin-
orbit coupled). Each of these calculation will have a different
optimized geometry.


Geometry optimization and NMR chemical shifts of ^1H in the H-X series
======================================================================

First we draw HF and define the settings for a geometry optimization
with the PBE functional and the ZORA scalar relativistic Hamiltonian,
using an all-electron QZ4P basis set with a ‘Numerical Quality’ good:

   **1.** Start up **AMSinput**
   **2.** Press 'h' to switch to the building mode and click somewhere in the canvas to draw an H atom
   **3.** Press ‘f’ and click somewhere close to the H to attach a fluorine atom to the H. Press 'Esc' to exit building mode.
   **4.** Symmetrize the system by clicking on [image: SymmTool][image]
   **5.** In the ADF Main panel, select the task **Geometry Optimization**
   **6.** Select the XC functional **GGA:PBE**
   **7.** Select the **QZ4P** Basis set
   **8.** Select the Frozen core **none**
   **9.** Select Numerical quality **Good**
 [image]

Note that typically for geometry optimizations a TZP or TZ2P basis set
is enough. For accurate NMR calculations one should use an all
electron basis set on the atoms on which one wants to calculate the
NMR parameters. Typically for NMR chemical shifts TZ2P is enough, but
if you can afford it one can use QZ4P, like in this case, see the FAQ
on recommend settings for NMR calculations.. A numerical quality good
is recommended for NMR calculations.

Now we will define the NMR calculation that will be performed after
the geometry optimization:

   **10.** In the panel bar, select **Properties →  NMR**
   **11.** Tick the **H atoms** box to apply the shielding for the H
   **12.** Select the two boxes to print the **Isotropic shielding constants** and **Full shielding tensors**
   **13.** Save the file with a name e.g. 'hf_pbe_scalar' (**File →  Save as**) and run the calculation (Ctrl/cmd + r)
 [image]

After the calculation is finished, you can look at the results:

   Open the logfile by clicking at **SCM →  Logfile**

You can find here at the bottom the NMR Shielding of 29.15 ppm. The
interatomic distance of HF can be found by, at first, reading in the
new coordinates and bonds after the geometry optimization. Then, if
you select both atoms, H and F, you can read the interatomic distance
from the geometry input box on the bottom right. You could also
extract the geometric data and NMR shielding in one go by using the
ADFreport tool (see below).


Run calculations for the other hydrogen halides
===============================================

Starting from HF, you can go down the periodic table by either making
new inputs, or changing the halide:

   Right-click on fluorine
   Select **Change Atom Type** and chose Cl for example

In the same manner, you can replace Cl by Br and then Br by I. Do not
forget to save the new molecule under a different job name, e.g.,
'hcl_pbe_scalar', 'hbr_pbe_scalar', etc.

These calculations will take increasingly more time to complete.
Collect the distances and NMR Shieldings.


Vary the relativistic approach and density functional (XC)
==========================================================

Now we will consider all the remaining combinations of the XC
functional and relativistic treatment. The most efficient way to
perform these subsequent calculations is to start with the converged
geometries for the PBE + SR (scalar-relativistic treatment) and change
the XC functional and/or the relativistic treatment.

   Navigate to the **Main** input panel
   Change the XC potential: **Hybrid** → **PBE0**
   Change the Relativity to **Spin-Orbit**

Make sure you save each new input file under a different name (e.g.,
'hcl_pbe_so', 'hcl_pbe0_scalar', etc.). Thus in total 4 calculations
will be performed for each molecule:

* XC functional (**GGA:PBE**) and Relativity (**Scalar**) (this
  calculation was done above)

* XC functional (**GGA:PBE**) and Relativity (**Spin-Orbit**)

* XC functional (**Hybrid:PBE0**) and Relativity (**Scalar**)

* XC functional (**Hybrid:PBE0**) and Relativity (**Spin-Orbit**)

The spin-orbit coupling NMR calculations will warn you that symmetry
will be switched off. Click OK to proceed. Hybrid calculations (PBE0)
will take longer than GGA calculations (PBE). Spin-orbit calculations
will take much longer than scalar relativistic ones.


Analyze multiple jobs with ADFreport
====================================

If you want to analyze the 16 calculations in one go, you can make use
of the AMSreport tool in AMSJobs (the window with your jobs listed):

   **1.** Open AMSJobs and select **Tools** → **New Report Template**
   **2.** Type in a name in the top-left entry (e.g. NMR and distance)
   **3.** Untick everything, tick the **NMR Shieldings** box (if you want other things reported, leave those ticks on)
   **4.** In the bottom field type **distance#1#2** to report the distance between atoms 1 and 2
   **5.** Click **OK** to save this report template
 [image]

   **6.** Select all 16 jobs
   **7.** Select **Tools** → **Build NMR and distance Report** (or whatever you called your report template)

You will get an html file reporting all distances and NMR chemical
shieldings in your default browser.

[image]

In this report zero's are shown for the NMR chemical shieldings on the
halide atoms that are not calculated.

These calculated numbers can be compared to experimental ones. The
experimental values for ^1H NMR chemical shifts δ_i can be found, for
example, in the paper by Wolff and Ziegler [1] . The NMR chemical
shift δ_i is defined as the difference of the isotropic NMR chemical
shielding of a reference compound σ_ref minus that of the interesting
compound σ_i:

δ_i = σ_ref - σ_i

In the paper the reference compound is HF, and σ_ref is the isotropic
value of the NMR chemical shielding of ^1H in HF (experimentally 28.72
ppm). With this definition of the reference compound the NMR chemical
shift of ^1H in HF is 0.0 ppm.

The experimental H-X distances can be found in the NIST Chemistry
Webbook , type in the molecular formula and look for the equilibrium
distance r_e.

The experimental numbers for the 4 compounds are:

+------------+-------------------------------------+-------------------------+
| Compound   | exp ^1H δ_i (ppm)                   | exp bond distance (Å)   |
|============|=====================================|=========================|
| HF         | 0.00 (definition)                   | 0.91680                 |
+------------+-------------------------------------+-------------------------+
| HCl        | -2.58                               | 1.27455                 |
+------------+-------------------------------------+-------------------------+
| HBr        | -6.43                               | 1.41443                 |
+------------+-------------------------------------+-------------------------+
| HI         | -15.34                              | 1.60916                 |
+------------+-------------------------------------+-------------------------+

The optimized geometries are close to the experimental ones. The
results with PBE0 are better than with PBE. The effect of spin-orbit
coupling on the optimized geometry is small for these closed shell
molecules, the largest effect can be seen for HI.

In the next picture the calculated ^1H NMR chemical shifts are
compared with the experimental ones. As one can see spin-orbit
coupling is needed to get reasonable agreement.

[image]


References
==========

[1] Wolff, S. K., and T. Ziegler. *Calculation of DFT-GIAO NMR shifts
    with the inclusion of spin-orbit coupling*. The Journal of
    chemical physics 109.3 (1998): 895-905.
