Section 25.2.4.3.3. The source code should not require customization for each project

The need to state this goal seems remarkable in these modern times, but the truth is that most computer programs in the 1970s required specific customizations before they could be used. The simplest modifications were the definitions of the maximum number of atoms, residues, atom types etc. accepted by the program. These modifications are still required in Fortran77 programs because that language does not allow the dynamic allocation of memory. However, in most programs today the limits are set high enough that the standard configuration does not present a problem.

The most difficult modification required for programs like PROLSQ was to adapt the calculations to the space group in hand. Their authors usually included code for the space groups they were particularly interested in, leaving all others to be implemented by the user. Writing code for a new space group was often a daunting task for someone who was not an expert programmer and had no tools for testing the modifications.

It is too burdensome to require the user to understand sufficiently the internal workings of a complex calculation that they can code and debug central subroutines of a refinement program. In its initial implementation, TNT avoided this problem, to an extent, by performing the space-group-specific calculations in separate programs. At least the user did not need to modify an existing program. All that was required was the construction of a program that read the proper format file, performed the calculation and wrote its answer in the proper format. The user was required to supply both a program that could calculate structure factors from the model and another program that could calculate the derivative of the diffraction component of the residual function with respect to the atomic parameters of the model.

While a structure-factor program could usually be located, either by finding an existing program or by expanding the model to a lower-symmetry space group for which a program did exist, the requirement of creating a derivative program proved too great a burden. The derivation of the space-group-specific calculation, its implementation and debugging proved too difficult for almost everyone, and this design was quickly abandoned. Instead, an implementation of Agarwal's (1978) algorithm was created. In this method, the derivatives are calculated with a series of convolutions with an map. The calculation of the map is the only space-group-specific part of the calculation, and this was done with a separate program for calculating Fourier syntheses. Such programs were as easy to come by as structure-factor calculation programs and could be replaced by a lower-symmetry program if required.

While it is easier to find or write a program that only calculates a Fourier transform and much easier to debug one than to debug a modification to a larger and more complex program, it is still difficult. The lack of availability of programs for the space group of a crystal often prevented the use of TNT. Over time, programs for more space groups were written and distributed with TNT. Eventually, a method was developed by one of TNT's authors in which FFTs could be calculated using a single program as efficiently as the original space-group-specific programs. Once this program existed, there was no longer the need for isolated structure-factor and Fourier synthesis programs. These calculations have disappeared into the heart of TNT, and TNT consists of many fewer programs today than in the past.