Performing a parallel relativistic calculation of EPR hyperfine coupling constant (A-tensor) is a simple two-step process

respect --scf  --inp=my-input-file --nt=4 --scratch=/path/to/scratch/directory
respect --hfcc --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 A-tensor calculation. The second call of respect with the argument --hfcc actually performs the EPR A-tensor 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 hfcc: with some EPR A-tensor-specific keywords. A comprehensive list of all keywords can be found for SCF here and for EPR A-tensor here. A simple example of my-input-file.inp looks like

#SCF input block
#4c Dirac-Kohn-Sham DFT (B3LYP) calculation of GaO molecule
scf:

    geometry:
        Ga   0.000    0.000    0.000
        O    0.000    0.000    1.744 

    method: mdks/b3lyp
    basis:  ucc-pvdz

    charge:        0
    multiplicity:  2
    maxiterations: 30
    nc-model:      gauss  #nuclear charge model


#A-tensor input block 
hfcc:
  active-atoms: all    #eval. A-tensor for all atoms
  nmm-model:    point  #nuclear magnetic moment model

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 calculations of A-tensor?

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

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

Yes. To scale the SO interaction set soscale: option in the SCF calculation. This setting is then automatically transferred to the HFCC calculation.

Q: Is there a way to launch SCF and EPR 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.