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# Calculation Of Indirect Spin-spin Coupling Constants

There are four contributions to the indirect spin-spin coupling constants, namely

a) the Fermi-Contact contribution J_FC

b) the Spin-Dipole contribution J_SD

c) the Paramagnetic Spin-Orbit contribution J_PSO

d) the Diamagnetic Spin-Orbit contribution J_DSO.

Calculation of one of these contribution is requested via

PROP=J_FC (--> FC contribution)

PROP=J_SD (--> SD contribution)

PROP=J_SO (--> DSO and PSO contribution)

and is possible for all quantum chemical schemes, for which analytic second derivatives have been implemented for UHF reference functions.

Note that, as the FC and the SD perturbations are spin-dependent, the calculation of these two contributions have to be currently performed using a UHF formalism, i.e., necessitates specification of REF=UHF even for closed-shell molecules. on the other side, calculation of the spin-independent SO contributions can be performed with a closed-shell reference.

As calculations of indirect spin-spin coupling are often hampered by triplet instabilities, the corresponding correlated calculations are best performed using an orbital unrelaxed approach. The latter is invoked by specifying DIFF_TYPE=UNRELAXED.

If only a subset of indirect spin-spin coupling constants is needed, the calculations can be restricted such that only for specific nuclei the corresponding FC, SD, or SO perturbations are considered (note that this means that all indirect spin-spin coupling constants involving this nucleus or these nuclei are after calculations available indepedent of whether the second nucleus is considered or not). The to be considered nuclei are specified via

 %j_info nucleus1 nucleus2 nucleus3 ...


after the normal ZMAT input with atom1, atom2, atom3, ... giving the ZMAT number of the corresponding nuclei.
Note that in this way it is possible to render otherwise too expensive calculations possible.

Examples