The calculation of hyperfine coupling tensors (in vacuum or in solution) is invoked by the keyword *ESR in the **RESPONSE input module. Thus a complete input file for the calculation of hyperfine coupling tensors will be:
**DALTON INPUT .RUN RESPONSE **INTEGRALS .FC .SD **WAVE FUNCTIONS .HARTREE-FOCK **RESPON .TRPFLG *ESR .ESRCAL .MAXIT 30 .TRPPRP FC Cl 01 .TRPPRP . . . .TRPPRP SD 01 x .TRPPRP SD 01 y .TRPPRP SD 01 z .TRPPRP . . . *END OF
This will invoke the calculation of hyperfine coupling tensors using the Restricted-Unrestricted methodology [46]. In this approach, the unperturbed molecular system is described with a spin-restricted MCSCF wave function, and when the perturbation - Fermi Contact or Spin Dipole operators - is turned on, the wave function spin relaxes and all first-order molecular properties are evaluated as the sum of the conventional average value term and a relaxation term that includes the response of the wave function to the perturbations.
The selection of a flexible atomic orbital basis set is decisive in these calculations. Dunning's cc-pVTZ or Widmark's basis sets with some functions uncontracted, and one or two sets of diffuse functions and several tight s-functions added have been shown to provide accurate hyperfine coupling tensors [47].
If more close control of the different parts of the calculation of hyperfine coupling tensors is wanted, we refer the reader to the sections describing the options available.