Performing a parallel relativistic calculation of NMR chemical shielding (CS) constant (σ-tensor) is a simple two-step process

respect --scf --inp=my-input-file --nt=4 --scratch=/path/to/scratch/directory
respect --cs  --inp=my-input-file --nt=4 --scratch=/path/to/scratch/directory

The first call of respect with the argument --scf performs the self-consistent field (SCF) optimization of ground-state molecular orbitals, which is a necessary prerequisite for the following σ-tensor calculation. The second call of respect with the argument --cs actually performs the NMR shielding constant simulation.

The other arguments mandatory to respect are

• --inp
  specifies a name of the input file;

• --nt
  specifies the number of physical cores required for parallel execution;

• --scratch
  specifies a path to the scratch directory.

The input file my-input-file.inp should contain the input block scf: with some SCF-specific keywords, and the input block cs: with some NMR σ-tensor-specific keywords. A comprehensive list of all keywords can be found for SCF here and for NMR shielding constant here. A simple example of my-input-file.inp looks like

#SCF input block
#4c Dirac-Kohn-Sham DFT (PBE0) calculation of HBr with Dirac--Coulomb Hamiltonian
scf:

    geometry:
       H      1.41400    0.00000    0.00000
       Br     0.00000    0.00000    0.00000

    method: mdks/pbe0
    basis:
            H:  upcS-1
            Br: dyall-vdz

    charge:        0
    multiplicity:  1
    maxiterations: 30
    convergence:   1.0e-6
    nc-model:      gauss   #nuclear charge model


#input block for the NMR shielding constant calculation with RMB-GIAO method
cs:

    maxiterations: 30
    gauge:         giao
    convergence:   1.0e-5

As a final note, there are several important and worth-to-remember aspects associated with the input syntax, namely

• the input is case-insensitive
  This means that the program does not distinguish between uppercase and lowercase letters.

• the input is insensitive to the number of blank lines and/or comment lines
  All comments begin with the number sign (#), can start anywhere on a line and continue until the end of the line.

• the input is compliant with the dictionary syntax of the YAML markup language
  This means that each input line is represented either by a single block: statement or by a simple keyword:value pair, such as

block1:
    keyword1: value1 
    keyword2: value2  
    ...
    block2: 
      keyword3: value3
      keyword4: value4
      ...
    block3: 
           keyword5: value5
           keyword6: value6
           ...

It is essential to remember that all members of one block: are lines beginning at the same indentation level. Whitespace indentation is used to denote the block structure; however, tab characters are never allowed as indentation. The only exception to the YAML-based input syntax is the block geometry: which utilizes a simple xyz format for the molecular geometry specification.

Q: How to scale the speed of light in NMR calculations?

Set the cscale: option in the SCF calculation. The scaling value is then automatically transferred to the CS calculation.

Q: Is it possible to scale the spin-orbit (SO) interaction in NMR calculations?

No. Currently one can only turn off the SO interaction by setting soscale: option to zero in the SCF calculation. This setting is then automatically transferred to the CS calculation.

Q: Is there a way to launch SCF and NMR calculations without the need to explicitly setup the scratch path by "--scratch=/path/to/scratch/directory"?

Yes, the argument "--scratch=/path/to/scratch/directory" can be saved to the file .respectrc in your home directory. If both the file and the command line argument exist, then ReSpect takes the scratch directory setting from the command line.

Q: How to run multiple NMR calculations, which will start from the same four-component molecular orbitals? Is there a way to avoid recalculating the 4c scf job multiple times?

One can run the --cs calculation with the command

/path/to/ReSpect/respect --cs --inp=cs-input --nt=4 --start-data=4c

where the last argument --start-data refers to the file with converged SCF molecular orbitals (in this example to 4c.50). In other words, the above command will take the initial SCF data from 4c.50 file and it will perform the --cs calculation according to the input cs-input.inp file. Now, it is sufficient if the cs-input.inp file contains only the cs: input block, for example

cs:

    maxiterations: 30
    gauge:         giao
    convergence:   1.0e-5

After successful end of the NMR job, the final NMR output data will be saved in the file 4c-cs-input.out_cs. This step can be repeated multiple times with different NMR input files but always starting from the same SCF molecular orbitals.