Indirect nuclear spin-spin coupling constants

As mentioned in the introduction of this chapter, the calculation of indirect nuclear spin-spin coupling constants is a time consuming task due to the large number of contributions to the total spin--spin coupling constant. Still, if all spin--spin couplings in a molecule are wanted, with some restrictions mentioned below, the input will look as follows:

**DALTON INPUT
.RUN PROPERTIES
**WAVE FUNCTIONS
.HF
**PROPERTIES
.SPIN-SPIN
**END OF DALTON INPUT

This input will calculate the indirect nuclear spin-spin coupling constants between isotopes with non-zero magnetic moments and a natural abundance of more than 1% . This limit will automatically include proton and $^{13}$C spin-spin coupling constants. By default, all contributions to the coupling constants will be calculated.

Often one is interested in only certain kinds of nuclei. For example, one may want to calculate only the proton spin-spin couplings of a molecule. This can be accomplished in two ways: either by changing the abundance threshold so that only this single isotope is included (most useful for proton couplings), or by selecting the particular nuclei of interest.

For a SOPPA or SOPPA(CCSD) calculation of indirect nuclear spin-spin coupling constants the additional keywords .SOPPA or .SOPPA(CCSD) have to be specified in the **PROPERTIES input module. For SOPPA an MP2 calculation has to be requested by the keyword .MP2 in the **WAVE FUNCTIONS input module, whereas for SOPPA(CCSD) a CCSD calculation has to be requested by the keyword .CC in the **WAVE FUNCTIONS input module with the *CC INPUT option .SOPPA(CCSD).

All the keywords necessary to control such adjustments to the calculation is given in the section describing the input for the *SPIN-S submodule. An input in which we have reduced the abundance threshold as well as selected three atoms will look as:

**DALTON INPUT
.RUN PROPERTIES
**WAVE FUNCTIONS
.HF
**PROPERTIES
.SPIN-S
*SPIN-S
.ABUNDA
 0.10
.SELECT
    3
    2    4    5
**END OF DALTON INPUT

We refer to the section describing the *SPIN-S input module for the complete description of the syntax for these keywords, as well as the numbering of the atoms which are selected.

We also notice that it is often of interest to calculate only specific contributions (usually the Fermi-contact contribution) at a high level of approximation. Sometimes the results obtained with a Hartree-Fock wave function may help in predicting the relative importance of the different contributions, thus helping in the decision of which contributions should be calculated at a correlated level [75]. The calculation of only certain contributions can be accomplished in the input by turning off the different contributions by the keywords .NODSO, .NOPSO, .NOSD, and .NOFC. We refer to the description of the *SPIN-S input module for a somewhat more thorough discussion.

If a closer a control of the individual parts of the calculation of indirect nuclear spin-spin coupling constants is wanted, this can be done through the use of keywords in the following input modules:

*EXPECT

Controls the calculation of one-electron expectation value contribution to the diamagnetic spin-spin coupling constants.

*GETSGY

Controls the set up of the right-hand sides (gradient terms).

*LINRES

Controls the solution of the singlet magnetic response equations.

*TRPRSP

Controls the solution of the triplet magnetic response equations (for Fermi-contact and spin-dipole contributions).

*RELAX

Controls the multiplication of solution and right hand side vectors into relaxation contributions.

*SPIN-S

Controls the choice of nuclei for which the spin-spin coupling constants will be calculated, as well as which contributions to the total spin-spin coupling constants are to be calculated.