Truncation method for shell model calculations

A method of truncating the large shell model basis is outlined. It relies on the order given by the unperturbed energies of the basis states and on the constancy of their spreading widths. Both quantities can be calculated by a simple averaging procedure. The method is tested in the sd shell where the JT dimensions are of the order of a few thousand. It proves to be very effective in the middle of the fp shell where JT dimensions of the order of a few times ${10}^6$ are truncated to a few times ${10}^3$.