F Form or scattering factors

ITEMS READ:

F , a scattering-factor-label, an integer FTYP giving the type of factor, followed by more numbers, depending on the type.

FTYP = 0

read 1 number to be a simple multiplicative factor.

FTYP = 1

read 1 number to be a neutron nuclear scattering factor (this is distinct from FTYP=0 because FTYP=0 may occur in addition to another type for the same atom, e.g. FTYP=2).

FTYP = 2

read 5, 7 or 9 numbers to be coefficients in an exponential approximation to the scattering factor.

FTYP = 3

read a table of pairs of numbers, S and f , being a table of values of the factor, f , tabulated versus S = .

FTYP = 4

as FTYP=2 except that each term of the expansion is multiplied by $(\sin\theta/\lambda)^2$. This type is provided for radial integrals $<j_l>$ with $l\ne0$.

FTYP = 5

Read radial wave functions from W RADF cards and use them to calculate the form factor.

FTYP =$-$1

read 2 numbers to be $f'$ and $f''$ of an anomalous scattering factor.

DETAILS:

Every F card has the general format F , name, type, some number of real numbers. If there is not room for all the necessary numbers on the first card, other similar cards may follow. But all F cards must start F name, type.

For types 0,1 and $-$1, one card will suffice for one factor. For type 2 the 5, 7 or 9 coefficients in the expression:

$$a_1 exp(-a_2S^2) + a_3 exp(-a_4S^2) + etc + a_n$$

are given; it will usually be possible to fit them on to one card. (Reference: International Tables Vol. C Sections 4.4.5 and 6.1.1.4, I.U.Cr 1992).

For type 3 you will almost certainly need more than one card. The values of S =  must be in ascending order, but need not be at equal intervals.

The label given on an F card will usually match one on an A card (q.v.). A warning is given if no A card matches at all, but this may be intentional.

Note that there is an implicit assumption in some of the library routines that scattering lengths and scattering factors are given in units of $10^{-12}$ cms. Using other units may give anomalous results for extinction corrections and when combining nuclear and magnetic scattering.

EXAMPLES:

F N14 0 100.

(a multiplying factor in addition to a form factor)

F Fe 1 0.951

(note that the 0.951 differs from the numbers given on F cards other than type 0, because it is allowed to be a refinable parameter)

F OXYG 2 3.048 13.277 2.287 5.701 1.546 0.324 0.867 32.909 .251
F Cr4 3 0 1 .05 .9738 .10 .9 .15 .7917 .2 .6655 .25.5372
F Cr4 3 .35 .3156 .4 .2309 .45 .1638 .5 .112 .6 .0442

ROUTINES WHICH READ THE CARDS:

Routine SETFOR calls INPUTF to read an F card which it then links to the atoms to which it belongs. (Routine SETFC calls SETFOR).