Dear Yunyun, I don't think Mike has written anything particularly incorrect , but he didn't point out why this approach is necessary. I believe there are two things you should definitely be aware of. An EXAFS dataset has a very limited amount of information. Approximately deltak.deltaR no of variables can be fitted To any spectrum as an absolute maximum and preferably quite a lot fewer than this (where deltak and deltaR are your fitted data range in k and R space respectively. Because of this if say you know you have N C atoms at ca G angstroms around your central atom. It is best not to use a variable for fit the degeneracy for that path but set it at the number you know is correct. And secondly because an EXAFS spectrum is effectively a collection of Sine waves there is a limited resolution, i.e. you cannot resolve paths of the same atom type that are less then Pi/2k(max) A apart, where k(max) is the maximum range of your dataset in k space, and Pi is Pi. Hence to separate two paths of the same atom that are 0.1 A apart you need about 15k worth of data. Regards Fred -- This e-mail and any attachments may contain confidential, copyright and or privileged material, and are for the use of the intended addressee only. If you are not the intended addressee or an authorised recipient of the addressee please notify us of receipt by returning the e-mail and do not use, copy, retain, distribute or disclose the information in or attached to the e-mail. Any opinions expressed within this e-mail are those of the individual and not necessarily of Diamond Light Source Ltd. Diamond Light Source Ltd. cannot guarantee that this e-mail or any attachments are free from viruses and we cannot accept liability for any damage which you may sustain as a result of software viruses which may be transmitted in or with the message. Diamond Light Source Limited (company no. 4375679). Registered in England and Wales with its registered office at Diamond House, Harwell Science and Innovation Campus, Didcot, Oxfordshire, OX11 0DE, United Kingdom