| Direct Methods | Patterson Interpretation | Partial Structure Expansion |
|---|---|---|
|
TITL... CELL... ZERR... LATT... SYMM... SFAC... UNIT... TREF HKLF 4 |
TITL... CELL... ZERR... LATT... SYMM... SFAC... UNIT... PATT HKLF 4 |
TITL... CELL... ZERR... LATT... SYMM... SFAC... UNIT... TEXP atoms HKLF 4 |
Although these standard settings should be appropriate for a wide range of circumstances, various parameters may be specified for TREF, PATT or TEXP and further instructions may be included between UNIT and HKLF for 'fine tuning' in the case of difficult structures. The parameter summary printed out after the data reduction in every job should be consulted before this is attempted, since the default settings for parameters that are not specified depend on the space group, the size of the structure, and the parameters that are actually specified (this is sometimes referred to as 'artificial intelligence' !).
All instructions commence with a four (or less) letter word (which may be an atom name); numbers and other information follow in free format, separated by one or more spaces. Upper and lower case input may be freely mixed; with the exception of the text strings input using TITL it is all converted to upper case for internal use in SHELXS. The TITL, CELL, ZERR, LATT, SYMM, SFAC and UNIT instructions must be given in that order; all remaining instructions, atoms, etc. should come between UNIT and the last instruction, which is almost always HKLF (to read in reflection data).
Defaults are given in square brackets in this documentation; '#' indicates that the program will generate a suitable default value based on the rest of the available information. Continuation lines are flagged by '=' at the end of a line, the instruction being continued on the next line which must start with at least one space. Other lines beginning with one or more spaces are treated as comments, so blank lines may be added to improve readability. All characters following '!' or '=' in an instruction line are ignored, except after TITL or SYMM (for which continuation lines are not allowed). AFIX, BASF, EQIV, EXTI, HFIX, RESI, SHEL, SIZE, SPAG, SPEC, TEMP, VOID, WGHT and XHAB (SHELXL) instructions may be present in the .ins file but are ignored.