X-ray absorption spectroscopy (XAS) is a powerful method for obtaining electronic and structural information around a well-defined absorbing atom site of different types of matter, from biological systems to condensed materials in almost all possible thermodynamics conditions. The low energy part of the XAS spectrum, from the rising edge up to few hundreds eV, the so called XANES (X-ray absorption near-edge structure) region, is extremely rich of electronic and structural information, like, for example, the oxidation state, overall symmetry, interatomic distances and angles. In this paper we present in details the MXAN method and a new reengineered release of the code proposed in the literature some years ago which allows a complete fit of the XANES energy region in term of well-defined set of structural parameters. This approach is based on the comparison between experimental data and many theoretical calculations performed by varying selected structural parameters starting from a putative structure, i.e. from a well-defined initial geometrical configuration around the absorber. The X-ray photo-absorption cross sections are derived using full multiple-scattering theory, i.e. the scattering path operator is calculated exactly without any series expansion. In this way the analysis can start from the edge without any limitations in the energy range and polarization conditions. Here we present in detail the theoretical background behind the code and new capabilities implemented from theory in this last version of the program as the analysis of XANES data coming from time-dependent spectra. The code is provided with modern technology for rapid deployment of binaries based on Docker runtime so that, without source compiling, its execution reduces to just a command line striking for a full parallel (shared memory) run even using complex construct like memory filesystem in a transparent way to the end-user.

Program summary

Program Title: MXAN

CPC Library link to program files: https://doi.org/10.17632/4k9363vcvh.1

Licensing provisions: GPLv3

Programming language: FORTRAN90

External routines/libraries: MKL or OpenBLAS library

Nature of problem: In MXAN (Minuit XANES) program we implemented a fitting procedure based on a full Multiple Scattering (MS) theory [1] able to extract local structural information around the absorbing atom from experimental XANES data. The MXAN method has been successfully used for studying many known and unknown molecular systems, yielding structural geometries and metrics comparable to X-ray diffraction and/or EXAFS results [2], [3], [4]. The release of this new code paves the route to new possibilities available now in the latest version of the program, as the analysis of XANES data coming from time-dependent and structural disordered systems.

Solution method: MXAN program is based on the comparison between experimental data and iterative theoretical calculations performed by varying selected structural parameters starting from a well-defined initial geometrical configuration around the absorber. The calculation of XANES spectra is performed within the so-called full MS approach, i.e. the inverse of the scattering path operator is computed exactly, avoiding any “a-priori” selection of the relevant MS paths [5], [6]. The fit procedure is performed in the energy space without the use of any Fourier transform algorithm; polarized spectra can be easily analyzed because the calculation is performed using the full MS approach [1]. The optimization in the space of the parameters is achieved using the CERN-library MINUIT [7] routines minimizing the square residual.

Additional comments including restrictions and unusual features: Depending on the molecular system under study and on the operating runtime conditions the program may or may not fit into available host RAM memory. The present release of the code is limited to a single SMP machine by running in parallel using OpenMP and/or the multi-threaded version of MKL or OpenBLAS.

中文翻译：

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MXAN：XANES实验的从头开始结构定量分析的新程序

X射线吸收光谱法（XAS）是一种强大的方法，可用于在几乎所有可能的热力学条件下，从生物系统到冷凝材料，在不同类型物质的定义明确的吸收原子位点周围获取电子和结构信息。XAS频谱的低能量部分，从上升沿到几百eV，即所谓的XANES（X射线吸收近边缘结构）区域，电子和结构信息极为丰富，例如，氧化态，整体对称性，原子间距离和角度。在本文中，我们详细介绍了MXAN方法和几年前文献中提出的代码的重新设计版本，该代码可以根据定义好的结构参数集完全拟合XANES能量区域。该方法基于实验数据与许多理论计算之间的比较，这些计算是通过从假定的结构（即从吸收体周围定义明确的初始几何构型）开始更改选定的结构参数来执行的。X射线的光吸收截面是使用完整的多次散射理论得出的，即，散射路径算子的计算是精确的，没有任何级数展开。这样，分析可以从边缘开始，而对能量范围和极化条件没有任何限制。在这里，我们将详细介绍该代码的背后的理论背景以及该程序的最后版本中从理论上实现的新功能，作为对来自与时间有关的光谱的XANES数据的分析。

计划摘要

节目名称： MXAN

CPC库链接到程序文件： https : //doi.org/10.17632/4k9363vcvh.1

许可条款： GPLv3

编程语言： FORTRAN90

外部例程/库： MKL或OpenBLAS库

问题的本质：在MXAN（Minuit XANES）程序中，我们基于完全多重散射（MS）理论[1]实施了拟合程序，该理论能够从实验XANES数据中提取吸收原子周围的局部结构信息。MXAN方法已成功用于研究许多已知和未知的分子系统，产生的结构几何形状和量度可与X射线衍射和/或EXAFS结果相比[2]，[3]，[4]。该新代码的发布为该程序的最新版本中现在可用的新可能性铺平了道路，因为XANES数据的分析来自与时间有关且结构混乱的系统。

解决方法：MXAN程序基于实验数据与迭代理论计算之间的比较，该理论计算是通过从选定的吸收体周围良好的初始几何构型开始，通过更改选定的结构参数来执行的。XANES光谱的计算是在所谓的完整MS方法中进行的，即精确计算了散射路径算子的逆，从而避免了对相关MS路径的任何“先验”选择[5]，[6]。拟合过程在能量空间中执行，而无需使用任何傅立叶变换算法；偏振光谱很容易分析，因为计算是使用完整的MS方法进行的[1]。使用CERN库MINUIT [7]例程可实现参数空间的优化，从而使平方残差最小。

其他注释包括限制和异常功能：根据正在研究的分子系统和运行时条件，程序可能会或可能不会适合可用的主机RAM内存。通过使用OpenMP和/或MKL或OpenBLAS的多线程版本并行运行，该代码的当前发行版仅限于一台SMP计算机。