**Authors:** Jim Stewart, Jim Holden, Ruth Doherty,
George Davenport, Syd Hall and Howard Flack

**Contact: **Syd Hall,
Crystallography Centre, University of Western Australia, Nedlands 6907,
Australia

*STARTX creates or updates an Archive (bdf) with
all of the cell, symmetry, crystal and structure information that is known
prior to the solution of the structure, exclusive of reflection data.
Reflection data is added by subsequent calculations.*

STARTX starts, or
updates (in `upd` mode) the *Archive*
by storing the initial information about
the file history, any text descriptive of the nature of the structure, unit
cell parameters, the unit cell symmetry, the types and number of atoms
contained in the unit cell, and various experimental data. The bdf is in
essence a repository for known information about the structure under study, and
will be available to all subsequent calculations. Other calculations will read
this file and add, supersede or delete information as requested.

Cell information is supplied by the **cell** and
**cellsd** input lines. This information is stored in the
*lrcell:* (the macro name that identifies the

STARTX provides several approaches to entering
symmetry information. The most concise way is to use the explicit-origin space
group notation (Hall, 1981) with the **sgname** line. This notation
is listed in the supplied `symmap` file. Space group symmetry can also
be defined by entering a **latice** and/or **symtry**
or **spaceg** lines. For example the space group A2/a (15:c1
z-unique) may be entered as follows:

*or*

*or*

*or*

symtry x, y, z symtry -x, -y,-z symtry 1/2-x, -y, z symtry 1/2+x, y,-z symtry -x, 1/2-y, z symtry x, 1/2+y,-z symtry 1/2+x, 1/2+y,-z symtry 1/2-x, 1/2-y, z |

The number of molecules per cell is entered with the
**celmol** line. The default value is one molecule per asymmetric cell unit.
If this is not correct for your structure, use a **celmol** line - as this
greatly assists routines such as * PEKPIK*,

The contents of the
crystal unit cell is with **celcon** input lines and using internal data
(such as the dispersion values) stored for each atom type. A **celcon** line
*must* be entered for each atom species expected in the structure. The
number of atoms of this type expected in the unit cell should also be entered.
If the scattering factor name entered is the 'standard' element name, then
other atom data such as the atomic weight, the atomic number, the number of
electrons, the usual atomic bond radius, the usual Van der Waals radius, and
anomalous dispersion scattering terms will be supplied by STARTX automatically.
*NOTE that if the neutron scattering length is entered all subsequent
calculations will assume that neutron data is being processed.*

STARTX
reads tables stored in the file `atomtype.cif` for the automatic generation of atomic scattering factors. The
use of these tables is dependent upon the value of the atomic number and the
number of electrons derived from the scattering factor name *or* specified
explicitly in the **celcon** input lines. For example, a Cu^{+} atom
could be defined as either Cu+ (or CU+ or cu+ or Cu1+ -- the scattering factor
symbol is not case sensitive whereas the atom label which incorporates this
symbol is), or CU with fields 3 and 4 set at 29 and 28, respectively.

STARTX also reads `atomtype.cif` for
data to enable scattering factors to be automatically generated for
Xrays, electrons and neutrons. Automatic generation is overridden by data
entered on formgn, formfx and celcon lines. Scattering factors may be generated
for s values out to 6.0 using the method of Fox et al. (1989) A.C. A45, 786.
The Stewart, Davidson & Simpson H curve is now generated from the Int Tab
Vol 4 coefficients in Table 2.2B, as do all scattering factors. Neutron
scattering lengths come from Int Tab Vol 4 Table 2.6. Scattering factors may be
printed by entering the line **reset psta 4**

In a departure from rationality STARTX will permit dummy atom types entered with an atomic number of 0 and a voidflg: or 0 molar weight to be treated as valid types. Use this feature to shoot yourself in the foot in some later calculation like CRYLSQ or FC.

The nature of the radiation may be specified on the STARTX line. Here are the input options:

Radiation type | xrays | generate xray scattering factors |

neutrons | generate neutron scattering lengths | |

electrons | generate electron scattering factors | |

Scatt factor limits | smax s | max s value of generated sf <2.0> |

sinc t | incr s used for generated sf<0.02> | |

A scattering factor symbol (also referred to as the atom type
symbol) consists of a string of up to eight characters. If the user enters the
usual international chemical symbols on the **celcon** line, these will be
used to generate appropriate scattering factor data without the need to enter
the atomic number or number of electrons (see last section also).

Optionally reads data from the input archive bdf

Writes the new or updated file to the output archive bdf

STARTX :the P6122 test in simplest form CELL 8.53 8.53 20.37 90 90 120 sgname P 61 2 (0 0 -1) CELCON s 6 CELCON o 6 CELCON c 12 |

title BODY-CENTRED ACENTRIC CUBIC TEST CASE, I 4BAR 3D STARTX CELL 11 11 11 90 90 90 sgname I -4bd 2ab 3 CELCON s 48 CELCON o 36 CELCON c 16 exper 1 1.5418 *9 1.234 maxhkl 12 8 4 .001 .9 |

STARTX label p6122 test 21 june 1982 CELL 8.53 8.53 20.37 90 90 120 cellsd .01 .01 .01 .01 .01 .01 latice n p :alternative is sgname p 61 2 (0 0 -1) symtry x,y,z symtry -x,-y,1/2+z symtry y,x,1/3-z symtry -y,-x,5/6-z symtry -y,x-y,1/3+z symtry y,y-x,5/6+z symtry -x,y-x,2/3-z symtry x,x-y,1/6-z symtry y-x,-x,2/3+z symtry x-y,x,1/6+z symtry x-y,-y,-z symtry y-x,y,1/2-z CELCON s 6 *8 .350 .869 CELCON o 6 CELCON c 12 |

Hall, S.R. 1981.

*Acta Cryst.***A37**, 517-525.Mott, N.F. and Massey, H.S.W. (1965)

*The Theory of Atomic Collisions*. pp86-112 Oxford Clarendon PressXray Form Factors: International Tables Volume C Table 2.2B

Neutron Form Factors: International Tables Vol C Tables 2.6 pp 270-271

Electron Form Factors: International Tables Vol C Tables 2.4.6 A neutral International Tables Vol C Tables 2.4.6 B ionized

Modified Mott s<0.1: Peng, L.-M. and Cowley, J.M. (1988)

*Acta Cryst*,**A44**, 1-5Modified Mott s>2.0: Fox, A.G., O'Keefe, M.A. and Tabbernor, M.A. (1989)

*Acta Cryst*,**A45**, 786-793.