Making Other Least Squares Programs

The existing main programs can be adapted to refine different parameters, or to use a different calculated function in the refinement. Examples which already exist are:

MPLSQ, multipole LSQ, in which the scattering density is expressed as a sum of spherical harmonic (multipole) functions, and the coefficients of these functions may be refined. A new family 5 has been defined for them, as there may be different numbers of multipole coefficients for different atoms.

PALSQ, polarization analysis LSQ, in which the observations to be fitted are the components of the scattered neutron polarization for a given input polarization.

Recently (April 2001) some changes have been made to make it easier to introduce new parameters into a least squares refinement based on structure factors. The author of the new program must provide:

1. An augmented main program in which the new parameters are defined this should be based on SFLSQ. The names of the new parameters (A4) must be added to ISFWRD and their specifications (family, genus, and species) to ISWDSP
2. A subroutine called say DOXXX1 with 1 dummy agument MODE. When called with MODE=1 it should read all the data from the CDF which are needed to give values to the new parameters. When called with MODE=2 it should check that enough data have been read, and do any necessary tidying up. For every new parameter which can be refined there should be a corresponding integer pointer. This pointer will be set to the number of the of the variable to which ithe parameter is assigned, or zero if it is fixed. DOXXX1 should be called in the main program with MODE=1 just after SETFC, and with MODE=2 just before PARSSF.
3. A logical function called say DOXXX2 with two dummy arguments NN and MODE

   MODE = 1

   Determine symmetry constraints on new parameters NN is not used.

   MODE = 2

   Set Variable pointers to new parameters NN is the variable number

   MODE = 3

   Apply shifts, one parameter per entry. NN is not used

   MODE = 4

   Write new parameter card (one per entry) NN is the serial number of the letter on the card to rewrite

   MODE = 5

   Do any calculation required because of parameter changes at the end of a least squares cycle.

   MODE = 6

   Interpret new words used to fix or vary groups of parameters when they apply to a particular genus. Such words should be defined in the main program as belonging to families with negative numbers (<-10).

   MODE = 7

   Interpret new words used to fix or vary groups of parameters when they apply to all such parameters (all genera or genus not applicable).

4. A subroutine called say LXXCAL to calculate the structure factors and their derivatives with respect to all the structural parameters (Only needed if the new parameters modify the structure factor)
5. A subroutine called say CALXXX to calculate the measurement (GCALC) to be fitted, in terms of the structure factor and other experimental parameters. (Only needed if the new parameters change how this is done)
6. A logical function called say DFTXXX which will be called with the dummy arguments set to the family, genus and species of a parameter. It should return TRUE if the parameter is to be varied.

In the EXTERNAL declaration in the main program:

LFCALC should be replaced by LXXCAL
DOTHER should be replaced by DOXXX2
DEFALT should be replaced by DFTXXX

A call to CALXXX should replace that to CALCSF inside the least squares cycles. It has two dummy arguments: H giving the reflection indices, and the second LXXCAL being the structure factor calculation routine. If, as for magnetic structures, two types of structure factors are to be calculated a third parameter gives the name of the required routine. It should be declared EXTERNAL. For example for magnetic least squares the call becomes: call CALCMG(H,LFCALC,LMCALC)
LFCALC calculates the nuclear structure factors and their derivatives.
LMCALC calculates the magnetic structure factors and their derivatives.
CALCMG combines them into GCALC to match the observed quantity GOBS.

Note. It is convenient to group the main program and the special subroutines in a single file to be included in the main program section of the master file.