NMR Shielding Tensor Tutorial
Performing the relativistic NMR shielding tensor calculations in ReSpect requires the following sequence of steps
SCF(1c) → SCF(4c) → NMR
where the initial SCF(1c) step represents the self-consistent field (SCF) procedure based on a scalar-relativistic, one-component (1c) Hamiltonian and this step is primarily meant to provide a very good guess of initial molecular orbitals for subsequent fully relativistic calculations where both scalar- and spin-orbit corrections are included variationally. In the second SCF(4c) step, the actual relativistic molecular orbitals are determined by means of a relativistic four-component (4c) SCF involving Dirac—Coulomb Hamiltonian. Finally, NMR shielding tensors are evaluated in the last step (NMR), starting from the relativistic molecular orbitals obtained in the previous SCF(4c).
To perform the SCF(1c) calculation, execute the command
/path/to/ReSpect/respect --scf --inp=1c --scratch=/path/to/scratch/directory
where arguments mandatory to
starts the SCF procedure;
specifies a name of the input file;
specifies a path to the scratch directory.
A simple example of the input file 1c.inp for a scalar relativistic one-component DFT/PBE0 calculation of HBr with the Douglas–Kroll–Hess Hamiltonian looks like
#scf procedure with 1c scalar DKH2 Hamiltonian scf: geometry: H 1.41400 0.00000 0.00000 Br 0.00000 0.00000 0.00000 method: ks-dkh2/pbe0 basis: H: upcS-1 Br: dyall-vdz charge: 0 multiplicity: 1 maxiterations: 30 nc-model: point
Note that a comprehensive list of all SCF keywords can be found here.
Having the initial SCF(1c) step finished, let's move on to the next SCF(4c) step. In order to have the 1c molecular orbitals ready for a restart, we execute the linux command first
ln -sf 1c.50 4c.50
which soft-links the ReSpect checkpoint file 1c.50 generated in the previous SCF(1c) calculation to a new checkpoint file 4c.50. Now, we can perform the second SCF(4c) step
/path/to/ReSpect/respect --restart --scf --inp=4c --scratch=/path/to/scratch/directory
where the additional argument --restart enforces
respect to search for the initial molecular orbitals in the checkpoint file 4c.50. An example of the input file 4c.inp is
#4c SCF Dirac-Kohn-Sham DFT calculation of HBr #molecule with the 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 nc-model: gauss convergence: 1.0e-6 #NMR shielding tensor calculation with RMB-GIAO cs: maxiterations: 30 gauge: giao convergence: 1.0e-5
Here, we replaced the 1c DKH2 Hamiltonian
method:ks-dkh2/dft-functional by the 4c Dirac—Coulomb Hamiltonian
Having the SCF(4c) calculation finished successfully, let's perform the final NMR step by running the command
/path/to/ReSpect/respect --cs --inp=4c --scratch=/path/to/scratch/directory
where the input block
cs: takes control of the setup for the NMR shielding tensor calculation. A comprehensive list of all NMR shielding tensor keywords can be found here.
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:valuepair, 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.
TIPS & TRICKS
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 spin-orbit interaction in NMR calculations?
No. Currently one can only turn off 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 set the number of processors for parallel SCF and NMR calculations?
For OpenMP parallel calculations, the number of processors can be controlled from the command line by the argument --nt=N, where N ideally refers to the total number of physical cores of a machine. Thus, the command line for launching an OpenMP parallel SCF or NMR job reads
/path/to/ReSpect/respect --nt=N --scf --inp=my-input-file /path/to/ReSpect/respect --nt=N --cs --inp=my-input-file
Note, however, we have assumed here that the scratch path is setup through the file .respectrc (see the previous discussion).
Q: I want to run a multiple cs calculations starting from the same four-component molecular orbitals. Is there a way to avoid recalculating the 4c scf job multiple times?
Yes, one can run the cs calculation with command
/path/to/ReSpect/respect --cs --inp=cs-input --start-data=4c
where the cs-input.inp file contains only the
cs: input block
cs: maxiterations: 30 gauge: giao convergence: 1.0e-5
The above command will take the initial data from 4c.50 and performs the cs calculation according to the input in cs-input.inp. After successful end of the cs job, the final output data will be stored in the file 4c-cs-input.out_cs. This step can be repeated multiple times with different cs input files but always starting from the same SCF(4c).
Department of Chemistry
UiT The Arctic University of Norway
Tromsø, NO-9037 Norway