A principle advantage of the TNT package is its great flexibility. The method used to define standard values for the stereochemical restraints is general enough to handle almost any situation likely to arise, while remaining very simple to specify. Because of this flexibility TNT is ideal for restraining structures which contain co-factors or inhibitors. It is equally useful for structures containing nucleic acids.
An additional flexibility of TNT is its ability to restrain a model to types of information which its authors have not considered. The package is composed of separate programs each performing clearly defined tasks. To use the package with other forms of data you simply write programs which produce the value and first derivative of the functional term you wish to minimize. (You can increase the rate of convergence by supplying the second derivative if you so desire.) TNT is distributed with programs which allow a model to be restrained to noncrystallographic symmetry, experimentally determined phases, or to perform real-space or rotation-function refinement.
Fast Fourier transforms are used whenever possible to speed the computations required by the minimization. All these Fourier transforms are calculated using as much of the space group symmetry as possible to hasten execution. These programs perform admirably for all non-centrosymmetric space groups and most centrosymmetric ones. The increase in computational speed causes TNT to far outpace programs which do not use FFTs, such as PROLSQ and SHELX -- It usually runs 20 times faster.
The package can be ran on either OpenVMS or Unix computers. The major programs in the package have been run on the Compaq Alpha computers (both Unix and VMS), the SGI line of Irix computers, the Sun Solaris series, and Linux running on Pentium-like systems. They should run on any Unix workstation, barring bugs in the local FORTRAN compiler.