The GRPMAT file

The GRPMAT file contains information describing the symmetry of the system you are interested in. It does this by a set of $3 \times 3$ matrices. The set of matrices must be closed. All GRPMAT files must contain the identity matrix, and since the matrices must form a complete set, each matrix must have an inverse, which is in the set. NRLMOL works quicker if the elements of the matrices are all 1s and 0s, but will accept other numbers as elements of the GRPMAT matrices. In some cases it is necessary to realign your system so that you can express the write the GRPMAT matrices using 1s and 0s. For example, a benzene molecule in the xy plane can have a six fold symmetric GRPMAT file with matrices having elements equal to the $\cos$ and $\sin$ of 30 and 60 degrees. However, by aligning the benzene molecule in a different plane describe plane, perhaps include picture the GRPMAT matrices can be written using only 1s and 0s. The first line of the GRPMAT file contains the number of matrices contained within the GRPMAT file. NRLMOL will pick up if there is a different number of matrices in the file, and if the matrices do not form a complete set.