Calculation of the NMR second moment for materials with different types of internal rotation.

The measurements of the NMR second moment can reveal information about structural and dynamical details of the sample providing that analysis of the experimental results on the basis of the Van Vleck's formula can be performed. In this paper the method of calculating the Van Vleck's second moment for solids with complex internal motions is presented. The method is based on simulating any desired motion of atoms within a block of unit cells large enough to reflect the property of the macroscopic sample and calculating the second moment value averaged by this motion. The detailed description of this method is given for the case of rotation of molecules or groups of atoms. The algorithm of the computer program which performs calculation based on the described method is also presented. Examples of application of the described method are listed.