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EasySpin can read ORCA outputs. ORCA is a quantum chemistry program with extensive support for the calculation of EPR properties such as the g tensor, A tensors, electric field gradients and D tensors.

Here, we look at how you can import these ORCA-calculated tensors into EasySpin, using the function orca2easysin.

Calculating EPR properties using ORCA

In order to get EPR parameters from an ORCA calculation, you have to tell ORCA to calculate these parameters. Here is a very simple example. Let's generate an ORCA input file `hydroxyl.oif`

:

! UKS B3LYP 6-31G *xyz 0 2 O 0 0 0 H 0 0 0.98 * %eprnmr gtensor 1 Nuclei = all H {aiso, adip, aorb, fgrad, rho} end

To get EPR parameters, include the `%eprnmr...end`

block that specifies what you want ORCA to calculate. `gtensor 1`

instructs ORCA to calculate the g tensor. The `Nuclei`

line tells ORCA to calculate for all hydrogens the following properties: Fermi contact hyperfine coupling (`aiso`

), dipolar hyperfine coupling (`adip`

), orbital contribution to hyperfine coupling (`aorb`

), electric field gradient at nucleus (`fgrad`

), and spin density at nucleus (`rho`

).

For details about ORCA calculations, see the ORCA manual.

Next, you need to run the ORCA calculation. On the command prompt, type

orca hydroxyl.oif > hydroxyl.oof

This will generate a long output file `hydroxyl.oof`

in text format. Also, it generates a smaller file `hydroxyl.prop`

. This is a binary file containing all the calculated properties.

Importing the results of the ORCA calculation

You can now use EasySpin's function orca2easysin to read the ORCA output and generate a spin system structure for you. EasySpin will not parse through the long text output file, but will directly read the binary propery file `hydroxyl.prop`

. Here is how it works:

Sys = orca2easyspin('hydroxyl.prop')

Spin not provided in the ORCA hydroxyl.prop file. Assuming S = 1/2. Sys = S: 0.5000 xyz: [2x3 double] g: [2.0023 2.0065 2.0774] gFrame: [0.1744 0 0] Nucs: 'H' A: [-9.7731 -116.1546 -140.4956] AFrame: [2.9791e-12 1.5708 1.3965] Q: [-0.0866 -0.1350 0.2216] QFrame: [-2.9672 0 0]

`Sys`

is an EasySpin spin system structure. All the fields are in the required units (MHz for tensors, radians for Euler angles). `Sys`

is almost ready for use in EasySpin. The only additional information you need to supply is some line broadening, e.g. in `Sys.lwpp`

.

Sys.lwpp = 1; % mT Exp.mwFreq = 9.5; % GHz Exp.Range = [315 350]; % mT pepper(Sys,Exp);

Hyperfine and quadrupole data for isotope mixtures

If you look at the `Sys`

structure above, you will see that the `Sys.Nucs`

field contains an element (hydrogen), without specifying a specific isotope (1H or 2H). In this case, EasySpin simulates all the spectra of all isotopologues with significant natural abundance and combines the results.

Of course, the hyperfine values are isotope-specific. For example, in the absence of any isotope effects, the hyperfine values of 2H are about 6.5 times smaller than those of 1H. The same holds for nuclear quadrupole couplings: They also depend on the isotope.

EasySpin has two simple rules to decide which isotope of a given element the hyperfine couplings in `Sys.A`

and the nuclear quadrupole couplings in `Sys.Q`

refer to.

- The hyperfine coupling in
`Sys.A`

refers to the most naturally abundant among the isotopes with spin 1/2 or larger. - The nuclear quadrupole coupling in
`Sys.Q`

refers to the most naturally abundant among the isotopes with spin 1 or larger.

All conversions from these leading isotopes to the others are handled by EasySpin automatically.