For a d(3) configuration ion, a theoretical method for studying the inter-relationships between electronic and molecular structure is investigated by diagonalizing the 120 x 120 complete energy matrices. The theoretical results show the following: (i) the local distortion structure for the Cr3+ occupying the Al3+ sites I and II exhibits an expansion distortion and the EPR parameters (D,g parallel to,g perpendicular to) are in good agreement with the experimental results at different temperatures. (ii) The vertical bar D vertical bar values decrease with the increasing temperature. (iii) The sign of the zero-field splitting parameter D is negative for the two trigonal Cr 3+ centers I and II in guanidinium aluminum sulfate hexahydrate (GASH) at 295.5, 78.45 and 1.61 K. (c) 2007 Elsevier B.V. All rights reserved.