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1. Introduction

Phit will fit a sum of line shapes to a real data set. The line shapes and variable parameters used in the fit are specified by the user. The parameters are varied using a Levenberg-Marquardt non-linear least-squares minimization to best fit the input data. All information needed by Phit, including the data file name, the line shapes, and the parameter list, are supplied in a plain text input file called phit.inp.

Phit performs extensive error analysis including measuring the goodness of fit and estimating error bars on the variable parameters. This is intended to help the user interpret the fit and extract physically significant results from the data.

Many portions of this document are copied or modified from the document for Feffit, the XAFS fitting program written by Matt Newville for the UWXAFS package. Phit borrows several concepts and algorithms from that program. Chapter 5 in the Feffit document is about goodness of fit and error analysis and serves well as a description of those concepts for this program.

Phit, as well as this document, is intended to meet the needs of a sophisticated user as well as a first time user. Much of this document is a reference manual covering all of the features and functionality of the program. Also included are several worked examples. The first time user should read these carefully and work through them at the computer. Sample data files and input files are distributed with the program.


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