TERRIG , a computer program for analysing thermal ellipsoids and rigid‐body motions

A program written in Fortran IV for the analysis of thermal ellipsoids and rigid‐body motions is reported. The program calculates the thermal vibration ellipsoids of the atoms and performs the rigid‐body motion calculations according to the TL method. The validity of the rigid‐body assumption is checked by employing the thermal ellipsoid size, shape and orientation criteria of Burns, Ferrier & McMullan [ Acta Cryst. (1967), 22 , 623–629]. The present program may prove useful in cases where the number of independent thermal parameters of the atom is insufficient to enable the unambiguous determination of the 20 TLS parameters.