# Ippcs.m

Point electron PCS model fitting. This function fits the point electron model PCS to the experimental pseudocontact shift coordinates and values. It returns the point electron coordinate, the predicted susceptibility tensor, along with the standard deviations for both.

## Syntax

    [mxyz,chi,pred_pcs,s_mxyz,s_chi]=ippcs(nxyz,mguess,expt_pcs)


## Arguments

    nxyz     - nuclear coordinates as [x y z] with multiple rows,
at which PCS is to be evaluated, in Angstroms. If
a cell array of such specifications is supplied,
the average PCS over them is being used.

mguess   - initial guess for the unpaired electron coordina-
tes as [x y z], in Angstroms.

expt_pcs - pseudocontact shift in ppm at each nucleus.


## Returns

    mxyz     - optimized paramagnetic centre coordinates as [x y z],
in Angstroms.

chi      - optimized magnetic susceptibility tensor in cubic
Angstroms.

pred_pcs - predicted pseudocontact shift at each nucleus with
the optimized mxyz and chi, ppm.

s_mxyz   - standard deviations of paramagnetic centre
coordinates as [x y z], in Angstroms.

s_chi    - standard deviations of magnetic susceptibility
tensor elements in cubic Angstroms.


## Examples

Multiple examples of point model fits are provided in /nmr_paramag/calbindin and /nmr_paramag/carb_anh directories of the example set supplied with Spinach.

## Notes

A good initial guess for the electron location is essential for a successful fit. Point model fails for nuclei positioned closer than about 10 Angstroms to the paramagnetic centre.